UVa

The Valladolid University Online Judge (UVa) Problemset
https://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.

ShyGypsy Labs
Various tools for UVa problems.

uDebug
We are an enthusiastic community of competitive programmers who help each other out by answering questions on chat, providing hints and solutions to problems from several online judges, furnishing test input and sharing feedback. We do all this while having fun and fostering a sense of camaraderie!