Mathematica, 10 bytes

Checkbox[]

Assumes Mathematica's notebook environment.

The two states look like this:

Processing, 78 66 65 63 61 bytes

2 bytes saved thanks to @TuukkaX by using -a%2 instead of a%2*-1

2 bytes saved thanks to @TuukkaX by using a=-a

int a=1;void draw(){background(a);}void mousePressed(){a=-a;}

The checkbox alternates between being black and white.

Explanation

int a=1;                   //the variable that contains the state of the checkbox
void mousePressed(){a=-a;} //if the mouse is pressed, change the sign of a

Now there are many other alternatives like using an if-statement to process this, but then it gets weird and constantly changes the background while the mouse is being pressed. I first tried mouseButton, but then I would need more conditions and more code that will just end up much more verbose.

Now for mousePressed() to be called forever (otherwise it would just stop after the program has just started), we need a draw() function.

void draw(){               //loop infinitely
  background(a);           //set the sketch's background to -a%2
  //if a ==  1, the background colour is set to almost black
  //if a == -1, the background colour is set to white
}

A smaller checkbox would be 90 bytes (the checkbox is in the top left corner, so we can remove some mouse conditions):

int a=1;void draw(){fill(a);rect(0,0,8,8);}void mousePressed(){if(mouseX<9&mouseY<9)a=-1;}

Bash, 67 55 50 35 32 31 bytes

:()(read -p$[i^=1]
›?9h -sn6
:)

The code contains a CR byte (first newline) and a CSI byte (small right angle bracket with the Windows-1252 encoding). It requires xterm (or equivalent) and the ISO-8859-1 encoding (or similar).

The code defines a function named :, which alternately displays a white 1 or 0 in a black rectangle. There are exactly 24 differing pixels with xterm's default settings. (proof)

For an older version that toggles the color of the entire terminal, check revision 10 of this answer.

Thanks to @ais523 for suggesting xterm, which saves 4 bytes!

How it works

:(...) creates a function named : that executes ....

The read command does the actual magic.

• -p specifies an input prompt, which is printed to STDOUT.

  • $[i^=1] is an arithmetic expansion which XORs the variable i with 1. i may initially be unset; when this occurs, it will be treated as 0.

  • The carriage return places the cursor back at the beginning of the line.

  • ›?9h captures the mouse in supported terminals. Each click will send six characters to the terminal, namely ←[Mbxy, where ← represents the ESC byte (0x1b). ←[ indicates an escape sequence, M the mouse, b the button (0x20 + the button number), x the x coordinate of the click (0x20 + coordinate), and y the y coordinate.

  • -sn6 makes read silent (the mouse input won't be echoed to STDERR) and stops after reading exactly 6 bytes. It saves the input in the REPLY variable, but we're not interested in the output it produces.

Finally, once read finishes (after exactly one mouse click), : recursively calls itself, entering an infinite loop.

Creation and invocation

$ echo $':()(read -p$[i^=1]\r\x9b?9h -sn6\n:);:' > checkbox.sh
$ xxd -c 17 -g 1 checkbox.sh
0000000: 3a 28 29 28 72 65 61 64 20 2d 70 24 5b 69 5e 3d 31  :()(read -p$[i^=1
0000011: 5d 0d 9b 3f 39 68 20 2d 73 6e 36 0a 3a 29 3b 3a 0a  ]..?9h -sn6.:);:.
$ LANG=en_US xterm -e bash checkbox.sh

Output

HTML, 20 bytes

Don't know how valid this will be as an answer. HTML requires the tag to be closed to legally validate, but modern browsers will automatically close them so it's executable.

<input type=checkbox

AHK, 25 bytes

Gui,Add,Checkbox
Gui,Show

Output -

Python REPL, 45 bytes

from tkinter import*
Checkbutton(Tk()).pack()

Tcl/Tk, 21 byte

Golfed

grid [checkbutton .c]

GeoGebra, 10 bytes

Checkbox[]

Entered into the input bar.

Here is a gif of the execution:

Unix Shell (+x11-apps), 3 bytes

Disclaimer

The answer below is boring, borderline and "feels cheating" (at least to me).

Yet, it does not seem to violate any of the challenge rules "as written", or "default loopholes", so I'm posting it.

(If you can point a specific rule it breaks, please comment on !)

Golfed

xgc

The xgc program demonstrates various features of the X graphics primitives.

The xgc screen is actually filled with various checkboxes:

and "sticky buttons" (which qualify as checkboxes, under this challenge rules):

HTML with JavaScript - 136 124 120 115 113 111 109 79 bytes

<p style=height:20;border:solid onClick=t=this;t.innerHTML=(t.e=!t.e)?'x':''>

-2 with thanks to @KritixiLithos

Reduced to 79 after some fantastic golfing from @Jan. Although the new version fits the requirements I have kept the old version for professional pride reasons.

While it is perfectly valid to use built-ins, I thought it was more fun to create my own from scratch. Tested with Chrome and Firefox. border:solid is there for IE compatibility but I don't know if it is actually needed in newer IE versions - I don't have IE available to test.

The px in the height is there for the benefit of "Run code snippet" so is not included in the "real" byte count. It works fine in the browser without it.

<p style=height:20px;border:solid onClick=t=this;t.innerHTML=(t.e=!t.e)?'x':''>

Previous version:

<p id=d style="width:20px;height:20px;border:solid 1px"onClick="d.innerHTML=d.innerHTML=='x'?'':'x'"align=center>

Html + javascript, 32 30 25 chars

<p onclick=innerHTML^=1>0

Octave, 36 20 bytes

uicontrol('sty','c')

Thanks to @LuisMendo saved 16 bytes!

Excel VBA, 38 32 Bytes

-6 Thanks to pajonk for replacing [A1],[A1],9,9 with 0,0,9,9

Immediates Window function that inserts a checkbox of height=9 and width=9, at the cell A1 on the first sheet of the active Excel VBA project

Sheets(1).CheckBoxes.Add 0,0,9,9

Scratch, 2 blocks

Put this script in the default sprite (which comes with 2 costumes)

TI-Basic, 15 bytes

While 1:Shade(0,9:Input :ClrDraw:Input :End

Fairly straightforward. The checkbox covers approximately 40% of the screen and alternates between black and white. When checked, the screen will look something like this:

C#, 124 bytes

note: because of edits, the picture doesn't exactly represent what the code makes, the "checkBox1" isn't displayed with the current code.

using System.Windows.Forms;class P{static void Main(){var a=new Form();a.Controls.Add(new CheckBox());Application.Run(a);}}

ungolfed:

using System.Windows.Forms;
class P{
  static void Main(){
    var a=new Form();
    a.Controls.Add(new CheckBox());
    Application.Run(a);
  }
}

VBA (Word), 74 57 bytes

-17 thanks to Taylor Scott

Sub a()
f = a.FormFields.Add(Selection.Range, 71)
End Sub

Filename of the document should be a.

KV Lang, 8 bytes

CheckBox

assuming a really basic Python environment for kv lang to even run:

from kivy.app import runTouchApp
from kivy.lang import Builder
runTouchApp(Builder.load_string('CheckBox'))

This works basically because CheckBox is a Kivy widget included in a kivy.Factory as each widget from the standard library.

That simple string creates an instance of CheckBox and because of runTouchApp expecting a widget to run I can use Builder.load_string.

The background of load_string handles basically every kv string and if there is a single instance of a widget defined (e.g. CheckBox), then it is elevated to a root widget position, for example:

Application
    openGL Window
        root widget(mostly layout)
            main tree
            and some widgets
            ...

Ruby with Shoes, 16 characters

Shoes.app{check}

Sample output:

HTML with JavaScript, 46 33 28 bytes

<body onclick=innerHTML^=1>0

v2

<body onclick=this.innerHTML^=1>0

v1

<body onclick=t=this;t.innerHTML=t.e=!t.e||''>

This is a variant on @ElPedro's answer: Create a Checkbox . Got down to 28 bytes with the help of @Albert Renshaw It could be further reduced, but that would make it identical to Create a Checkbox

The whole application area is clickable. It will display 0 in the off state and 1 in the on state. Try it out at https://jsbin.com/nuhetiyocu/1/edit

Powershell v5+, 94 Bytes

($a=[Windows.Forms.Form]::new()).Controls.Add([Windows.Forms.Checkbox]::new());$a.ShowDialog()

This is pretty terrible, but I can't get the statements any shorter with either Add-Type or using namespace - and the variable is required, since I need to reuse the object to show the form, and Controls.Add() returns nothing usable.

ShowDialog is required on my system to have the box be interactive after running this command, otherwise the form is frozen.

also System.Windows.Forms.Form can only be shortened to Windows.Forms.Form before (at least my own) system stops auto completing it in a fresh session.

Any version of Windows command line, 3 bytes

(cmd.exe, batch, powershell, start->run, whatever)

osk

Opens the on-screen-keyboard which has several "checkboxes" such as shift, ctrl, alt, capslock keys Similar to the xgc answer, and inspired by the SmileBASIC answer screenshot.

AutoIt, 78 Bytes

GUICreate(0)
GUISetState(GUICtrlCreateCheckbox(0,0,0))
Do
Until GUIGetMsg()=-3

Opens a maximized window that can be closed like any other windows window. The maximization and ... on-top-ness is a side effect of the golfing.

Python, 62 bytes

from tkinter import*
a=Tk()
Checkbutton(a).pack()
a.mainloop()

SmileBASIC, 49 bytes

@L
O=T
TOUCH OUT T,,
X=T<O!=X
KEY 1,""[X]GOTO@L

 and  are an empty box and a checked box in SB's font.
Toggle the checkbox by touching the screen (the closest thing to clicking with a mouse)

Pug, 20 bytes

input(type=checkbox)

Clojure, 201 195 145 bytes

(ns c(:require[quil.core :as q]))(def a(atom nil))(q/defsketch b :draw #(apply q/background(if @a[0 0 0][255 0 0])):mouse-clicked #(swap! a not))

-6 bytes by getting rid of the need for m/fun-mode.

-50 bytes by removing the :setup and :size options, since they are defaultable and unnecessary in non-fun-mode. Also removed the unnecessary import for middleware.

Creates a Quil sketch (a Clojure library that provides a layer over Processing), therefore, this requires the Quil library to run.

(ns bits.golf.checkbox
  (:require [quil.core :as q]))

; Mutable global variable to hold the toggle state
(def a (atom nil))

(q/defsketch b
         ; Draw the background color to be either black or red depending on the toggle state.
         :draw #(apply q/background (if @a [0 0 0] [255 0 0]))

         ; Negate the state on mouse click
         :mouse-clicked #(swap! a not))

Slim, 21 Bytes

input type="checkbox"

Try it here

So yeah. One of slim's major features is that < and > characters are not there. So basically, I get 2 bytes less than the creator of the question!

Bash + zenity, 31

zenity --checklist --column a a

Bash (+yad), 23 bytes

Golfed

yad --form --field :chk

YAD is a program that will display GTK+ dialogs, and return (either in the return code or on standard output) the users input. This allows you to present information, and ask for information from the user, from all manner of shell scripts.

Yad is available in Debian and Ubuntu repositories (and probably others).

Demo