UVa

The Valladolid University Online Judge (UVa) Problemset
http://acm.uva.es/p

History
This site has been around for quite awhile, with over 3 million submissions as of December 15, 2004.

Related Links
felix-halim.net This site has many things related to the UVa Online-Judge, like a statistics comparer, a sorting of the easiest problems and separated ranklists for the countries.

Methods to Solve Compilation of various methods to solve the problems. Has methods for about 35% of the problems.