C#

• StyleCop (free)

  It can be run from inside of Visual Studio or integrated into an MSBuild project.

• ReSharper (commercial)

  A paid Visual Studio extension (possibility for open-source "community" license though) with a free 30-day trial.

• FxCop (free)

  Can be run standalone, but later versions are bundled into Visual Studio as the "Code Analysis" feature.

C / C++

• Clang's Static Analyzer, a free static analyzer based on Clang.

• Coverity, a commercial static analyzer, free for open source developers

• Cppcheck, an open-source static analyzer

• CppDepend, a commercial static analyzer based on Clang

• gcc

Website: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Simply compiling code with -Wall -Wextra -pedantic options catches quite a number of potential or actual problems.

VBA

• Rubberduck VBE Add-In is a free (beer & speech) add in for the VBA editor maintained by several members of the vba and c# community here on Code Review. For more info see the rubberduck tag.

  Direct Link to Code Inspection Documentation

JavaScript

For JavaScript, there are a few options:

• JSLint (the original, created by Douglas Crockford)
• JSHint (a community-driven fork of JSLint)
• ESLint (an ES6-compatible fork of JSHint. Necessary if you want to lint ES6 code).

In all cases, if you're using a transpiler like Babel or Traceur, you should run the linter before the transpile step. Don't lint compiled code.

Note that all major browsers and other interpreters support the "use strict"; directive in code, and will ensure that many common mistakes in code are identified before running the code. See: What does “use strict” do in JavaScript, and what is the reasoning behind it?

Python

Python uses Python Enhancement Proposals (PEPs) to enhance Python, ranging from style guides to feature requests. There are linter's for a few of these:

• PEP8 -- Style Guide for Python Code.

  There are both online(pep8online.com) and offline(pep8) versions for this.

• PEP257(pep257) -- Docstring Conventions.

• PEP484(mypy) -- Type Hints.

There is also "PEP7 -- Style Guide for C Code". But this does not seem to have a linter.

There are also some other analysis tools:

• pylint
• pyflakes

• pychecker

  Warning: This imports the code to analyse it.

• flake8

  Combines pyflakes and pep8 into one tool.

• Prospector

  Combines all of: pylint, pyflakes, pep8, pep257 and more tools.

There are also ways to incorporating some of the above tools into different editors. For a small list of incorporations:

• Sublime Text has SublimeLinter 3,
• vim has pylint.vim and pep8,
• atom has linter-pep8, and
• pycharm has pep8 by default.

These are good to make sure your answers are PEP8 complient, and so there are no contradictions when saying to follow PEP8. However using the tools from the above sections will give a report that you can comment from.

HTML5, XHTML, CSS

Tool: Markup validation checker

Also, there are beautifiers to help improve formatting, CodeBeautify.org has a good one.

VB.NET

• ReSharper (commercial)

  A paid Visual Studio extension (possibility for open-source "community" license though) with a free 30-day trial.

Delphi

Tool: FixInsight (commercial)

Website: http://sourceoddity.com/fixinsight/

Java

Java lint tools are usually dependent of the IDE you develop in. All the major IDE's (Eclipse, IntelliJ, Netbeans, etc. - alphabetical order) have mechanisms in place for not only checking for lint-like problems, but also for fixing them too.

Features to expect from your IDE - identification of, and correction of:

1. automatic code formatting (indentation, brace positions, line-wrapping, etc.)
2. redundant code, or impossible code
3. incomplete documentation
4. variable and function name "shadowing"
5. and much, much more.

Standalone tools:

1. PMD
2. FindBugs

PHP

Online:

• PHP Linter Online

• PHPCodeChecker.com

Plugins:

• NuSphere

You can also use the native php_check_syntax from an IDE or command line.

C

• splint

Although not currently actively developed, this free, open-source software provides error checking that can be enhanced with in-code commenting.

SQL

General:

• Program: Mimer SQL Validator

• Online: FreeFormatter.com

SQL Server / Transact-SQL

Online: SQL-Format.org

Natively: SET SHOWPLAN_XML

MySQL

Online: MySQL Syntax Check

Natively: EXPLAIN

PostgreSQL & PL/pgSQL

Plugin: pg-validator

Natively: EXPLAIN

SQLite

Plugin: sqllogictest

Natively: EXPLAIN

Oracle:

Natively: EXPLAIN

jQuery

Plugins:

• jQueryValidation.org

• FormValidator.net

You can also use JavaScriptLint.com.

Ruby

Gems:

• rubocop is probably the most popular

But there are many more each with emphasis on different things (style, code complexity, etc.).

Online:

• Don't know if this counts as it's more like CI, but there's always CodeClimate (also works for JavaScript, PHP, and Python)

Regex:

JS-Flavored

• Debuggex.com - online regex visual debugger

• Regexr.com - online regex tester

• Regex101.com - online tester and debugger

Offline-Tools:

• RegexBuddy - commercial regex-development helper tool, supporting a multitude of regex-flavors and programming languages

Objective-C

Plugins:

• OCLint.org

• Uncrustify

• linter (for Atom.io)

_{These should also work for C/C++}

XML

Online validators:

• XML Validation.com

• W3Schools.com

Plugins:

• Maven

• Grails

• Notepad++

• FireFox

Perl

Plugins:

• Perl::Lint

• linter-perl

• Perl::Critic

Or natively using B::Lint module.

Lisp

Plugins:

• LINT: Lint for Common Lisp

• clavier

Lua

Plugins:

• lua-checker

JSON

Online validator: JSONLint.com

They also have a free Pro Version

Scala

Style checker plug-in: ScalaStyle.org

Online evaluator: SimplyScala.com

Bash, Shell

Online:

• ShellCheck.net

Plugins:

• bashlint

• ShellCheck

MongoDB

Plugins:

• Mongoose (for Node.js)

• revalidator (for Node.js)

• lx-valid (for Node.js)

• Mongoid (for Ruby)

Or natively use validate