The Valladolid University Online Judge (UVa) Problemset
The old server, which doesn't receive submissions anymore:
This site has been around for quite awhile, with over 7 million submissions as of February 26th, 2009.
 Related Links
An archive of helper utilities, such as a statistics comparer, a sorting of the easiest problems and separated ranklists for the countries.
Compilation of various methods to solve the problems. Has methods for about 35% of the problems.
Inspired by Methods to Solve. Has solutions to over 530 UVa problems in a database searchable by keywords, algorithm types, and difficulty.
Various tools for UVa problems.
Various Tips for UVa problems.
Various tools for UVa problems. You can construct your own test cases and get the correct output (currently available for over 1430 problems). Also has keywords/classification for all these problems.
Solutions for UVA problems using C++ and Java
Sample algorithms, solutions and codes for UVA problems. Some solutions are presented with substantial explanations. Samples codes are written mostly in C/C++. Tutorials are given on how to parse and spot easy problems. Some simulations are also provided to enhance the understanding of applied algorithms.