Gareth Heyes wrote a nice blog entry on JavaScript hacks:

"I love to use JavaScript in unexpected ways, to create code that looks like it shouldn't work but does, or produces some unexpected behavior. This may sound trivial, but the results I've found lead to some very useful techniques. Each of the techniques described can be used for XSS filter evasion, which was my original intention when developing them. However, learning such JavaScript can dramatically increase your knowledge of the language, helping you become better at cleaning up input, and increase web application security.

So read on and enjoy my weird and wonderful JavaScript hacks."

http://dev.opera.com/articles/view/opera-javascript-for-hackers-1/