http://www.topcoder.com/tc?module=ProblemArchive
You can search for different types of contest problems, whatever suits your fancy, and you can also can adjust your search criteria to more difficult problems. If you're a stuck on a problem, you can look at other solutions which members have submitted to the problem in the topcoder client and you also look for the full detailed explanations of solution to the problems here http://www.topcoder.com/tc?module=Static&d1=match_editorials&d2=archive. When I found this stuff, I couldn't believe how something so awesome could have existed without me knowing, and that members in compsci.ca would want to know about this stuff too.
