I linked Haris through to the widely used Packer (from Dean Edwards). If your JavaScript didn't work after being 'packed' chances are there's some non-standard thing in your code which the packer trips up on -- semi-colons being the usual culprit (they're essential since packed code is all on one line).

Wildhoney mentioned using Prototype to cut down on the amount of code that you have to write. Remember that you can compress Prototype down as well, and if you're using more modular libraries/frameworks (Mootools is a good example) then it really makes sense to only ever include the parts of the libraries/frameworks that you're going to be using (if that's possible).

Other popular compression tools include YUI Compressor, Douglas Crockford's JSMin and Dojo ShrinkSafe. I don't know if any of those have a web interface to copy/paste code from/into.