generate a random ID

annotated.js

function(
  a, // the character length of the ID (optional, 32 by default)
  b  // the radix of the ID (optional, 16 by default)
){
  b = b || 16;         // use a default radix of 16
  return Array(        // return an array
    a                  // of the length, if specified,
    || 32              // or 32 otherwise,
  )                   
  .join(0)             // joined into a string of 0s,
  .replace(            // and then replace
    /./g,              // each one, with
    function() {
      return(         
        0|             // a rounded
        Math.random()* // random number between 0 and
        b              // the radix,
      )
      .toString(b)     // and serialize it into a string.
    }
  )
}

index.js

function(a,b){b=b||16;return Array(a||32).join(0).replace(/0/g,function(){return(0|Math.random()*b).toString(b)})}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Jed Schmidt <http://jed.is>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "randomID",
  "keywords": [ "random", "ID" ]
}

Save 5 bytes.

function(a,b){return(Array(a||32)+'').replace(/,/g,function(){return(0|Math.random(b=b||16)*b).toString(b)})}

And a bit more bytes cut-off:

function(a,b){var c='';a=a||32;while(a--){c+=(0|Math.random(b=b||16)*b).toString(b)}return c}

Shave more bytes off:
function(a,b,c){for(a=a||32;a--;c=[c]+(0|Math.random(b=b||16)*b).toString(b));return c}

Great work!