List of Problemsets
 Online judges with (mostly) ACM problems
- 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.
 Programming contests
- IOI - International Olympiad in Informatics - http://olympiads.win.tue.nl/ioi/, http://www.ioinformatics.org
- TopCoder - http://www.topcoder.com - Quasi-weekly programming competitions from algorithms to components to marathons.
- Canadian Computing Competition - A two-stage annual olympiad. See also this categorization of past Stage 1s by topic, with solutions.
- SPb IFMO Training Sessions - http://neerc.ifmo.ru/trains/ (Note, all problems appear to be in Russian)
- South African Computer Olympiad - http://olympiad.cs.uct.ac.za/old/ - Archive of finals and IOI training contests
- CodeChef - http://www.codechef.com - Ongoing monthly algorithm programming competitions and short weekend challenges with prizes. Also has practice problems ranging from easy to hard.
- URI Online Judge - http://www.urionlinejudge.com.br/ Programming practice site for students and teams with several problems divided into 8 categories covering several programming subjects
- Project Euler - http://projecteuler.net - Project Euler consists of challenging mathematical or computer science problems.
- Open Source Instititute - http://www.osix.net - Increasingly difficult "programming puzzles" to solve, including steganography, cryptography and plain-old algorithms.
- Python Challenge - http://www.pythonchallenge.com/ A series of cute puzzles to solve. You don't have to use Python (but some problems reference specific Python libraries)