UVa

The Valladolid University Online Judge (UVa) Problemset
http://uva.onlinejudge.org/

The old server, which doesn't receive submissions anymore: http://acm.uva.es/problemset

History
This site has been around for quite awhile, with over 7 million submissions as of February 26th, 2009.

Problems
Please use the Problem Template when adding new problems; see Help:Editing for a step-by-step guide.

felix-halim.net
An archive of helper utilities, such as 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.

ShyGypsy Labs
Various tools for UVa problems.

ACMSolver Site
Various Tips for UVa problems.

UVA toolkit
Various tools for UVa problems. You can construct your own test cases and get the correct output (currently available for around ~1400 problems). Also has keywords/classification for all these problems.