I would like to try my hand at hacking -- that is, exploiting various website vulnerabilities. Not for any illegal purpose mind you, but so I can have a better understanding and appreciation of these exploits while writing my own web software.

I seem to recall that there was a community that hosted a bunch of demo websites, and you had to find and exploit certain vulnerabilities with each one. I can't remember what it is called but this is the sort of thing I am looking for -- I have read a tonne of little XSS and CSRF examples but have yet to find a real-life hands-on example of one.

Does anyone know of such a place, where I can be given an example page and look for security holes? I would really rather not try this with actual websites, I don't want to break any laws.