Hacker war games are great fun and I've personally found Hack This Site! to be completely addictive when I've got some down time at work.  Its a great way to get a basic understanding of attacks w/o ending up in prison or the Dean's office.   You can watch my progress here : http://www.hackthissite.org/userinfo.php?id=akb    ....don't laugh I've only been at this off and on for two days ;)