HTML 21 7 chars (1 char if I'm cheeky...)

‮n

replace n with your number

Python

int(str(76543)[::-1])

EDIT:

Shorter solution as suggested by @gnibbler:

int(`76543`[::-1])

or, if above is unclear:

x=76543
int(`x`[::-1])

Universal (language agnostic/independent)

If you want to use only numbers (avoid converting the number to string) and don't want to use some specific library (to be universal for any language):

x = 76543 # or whatever is your number
y = 0
while x > 0:
    y *= 10
    y += ( x %10 )
    x /= 10 # int division 

_{This is python, but it could be done in any language, because it's just a math method.}

Perl 6

+$n.flip

or:

$n.flip

for dynamically typed code.

Numbers got string methods due to language design.

Haskell, 28 24 characters

f=read.reverse.show.(+0)

Vim

:se ri<CR>C<C-R>"

PHP, 9 chars

(int)strrev(123);

To do it short where N is a constant:

strrev(N)

J - 6 characters + variable

".|.":y

Where y is your value.

Ruby (14)

x = 13456
x.to_s.reverse

Python:

int(''.join(reversed(str(1234))))

Golfscript, 5 chars

`-1%~

This takes an argument on the stack and leaves the result on the stack. I'm exploiting the "subprogram" option in the spec: if you insist on a function, that's four chars more leaving it on the stack:

{`-1%~}:r

In shell scripting :

  echo "your number"|rev

Hope this was useful :)

It is possible to convert a number a string, then reverse the string and then convert that string back to number. This kind of feature is probably available in all language. If you are looking for a more mathematical method then this might help:

int n = 76543;
int r = 0;

while (n > 0) {
    r *= 10;
    r += n % 10;
    n /= 10;
}

Scala - 33 Chars

def r(a:Int)=(a+"").reverse.toInt

You could do the following in Java. Note that this converts to String and back and is not a mathematical solution.

public class test {
    public static int reverseInt(int i) {
        return Integer.valueOf((new StringBuffer(String.valueOf(i))).reverse().toString());
    }

    public static void main(String[] args) {
        int i = 1234;
        System.out.println("reverse("+i+") -> " + reverseInt(i));
    }
}

You can use reversing a String in Java

long l =
long reversed = Long.parseLong(new StringBuilder().append(l).reverse().toString());

Python 3+

Function form: 28 characters

r=lambda i:int(str(i)[::-1])

(Sub)program form: 25 characters

print(input()[::-1])

I consider some of the other Python examples to be cheating, or at least cheap, due to using hardcoded input and/or not fully satisfying the requirements.

Lua

Numbers and strings are interchangeable, so this is trivial

string.reverse(12345)

Actionscript

43 characters. num as the parameter to the function:

num.toString().split('').reverse().join('')

Java

""+new StringBuilder(""+i).reverse();

Groovy

r={"$it".reverse() as BigDecimal}

assert r(1234) == 4321
assert r(345678987654567898765) == 567898765456789876543
assert r(345346457.24654654) == 45645642.754643543

Perl, 11 chars

The p flag is needed for this to work, included in the count.

Usage:

$ echo 76543 | perl -pE '$_=reverse'

Clojure (42 chars)

#(->> % str reverse(apply str)read-string)

Example usage:

(#(->> % str reverse(apply str)read-string) 98321)

returns 12389

APL (3)

⍎⌽⍕

Usage:

⍎⌽⍕12345 => 54321

Befunge (3 characters)

Complete runnable program:

N.@

Where N is your number. Rules say "accept a single integer valued argument"; In Befunge you can only enter integers from 0 to 9.

Kinda late but

APL, 3

⍎⌽⍞

If you insists on a function

⍎∘⌽∘⍕

this is AS3 code, so you may need to make slight changes

function reverseData ( inData:String ):* {
    var ar1 = inData.split(''); //Takes string value, split each digit into an array

    var ar2 = ar1.reverse(); //Inverses the array direction 
    /* //OR (if not supported) [not AS3]
    var ar2 = new Array();
    for( var i:int = (ar1.length - 1); i > 0; i++ ) {
        ar2.push( ar1[i] );
    }
    */

    var result = ar2.join();
    return result;
}

It should work for a string equavalent (that should be easy to typecast)

For a javascript example...

function flip( inData ) {
    return ( parseInt( (inData + '').split('').reverse().join('') ) );
}

alert( flip(123) );

Depends on what you mean by short (javascript):

alert(String(123).split('').reverse().join('')),

C#

int reversed = Convert.ToInt32(String.Join<char>(null, 76543.ToString().Reverse()));

This one ACTUALLY takes an input, unlike some of the rest:

print`input()`[::-1]

Python btw.