I stumbled upon an interesting resource today - the XSS Cheatsheet. This is a really wonderful collection of XSS (cross-site scripting) test cases. If you don't know what XSS is, you might find the following resources helpful:
Christian has developed a script for filtering data specifically for XSS. He also has an example implementation where you can try it out for yourself - maybe someone with some free time can try entering all of the test cases to see if any of them expose a weakness.