The Algorithmist is a resource dedicated to anything algorithms - from the practical realm, to the theoretical realm. There are also links and explanation to problemsets.
UVa Problemset - http://acm.uva.es/p - The Valladolid University Online Judge. Over N problems, for a reasonable value of N. The problems are culled from old contests, and online contests.
ACM-ICPC Live Archive - http://acmicpc-live-archive.uva.es/nuevoportal/ - The 2000's ACM-ICPC Live Archive Around the World. Contains actual problems from regionals and finals from 2000 on.
Maintained by Larry, Ryan, Jason and Jack.