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.


 * Sphere Online Judge - http://spoj.sphere.pl


 * Zhejiang University Online Judge - http://acm.zju.edu.cn


 * Ural State University Online Judge - http://acm.timus.ru


 * Saratov State University Online Contester - http://acm.sgu.ru


 * Peking University Judge Online of ACM ICPC - http://acm.pku.edu.cn/JudgeOnline


 * El Judge Online MIPT programming contest - http://acm.mipt.ru/judge


 * Programming Challenges - http://www.programming-challenges.com


 * Arbiter - http://acm.sharif.edu - Sharif University's Online Judge System

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.


 * USACO Contest - http://ace.delos.com/contestgate - Monthly contests (Nov - March)


 * Canadian Computing Competition - A two-stage annual olympiad. See also this categorization of past Stage 1s by topic, with solutions.


 * IPSC - http://ipsc.ksp.sk


 * BitWISE - http://www.bitwise.iitkgp.ernet.in


 * BUTE International 24-hour Programming Contest -http://www.challenge24.org


 * SPb IFMO Training Sessions - http://neerc.ifmo.ru/trains/ (Note, all problems appear to be in Russian)


 * UPC2001 Internet Contest - http://contest.psu.ru/Inet (link currently broken)


 * Moscow SU Virtual Contests (in Russian) - http://acm.msu.ru/virtual


 * Al Zimmermann's Programming Contest - http://www.recmath.org/contest/index.php

Other

 * USACO Training - http://ace.delos.com/usacogate - "Training" site for the USACO project


 * Project Euler - http://mathschallenge.net/index.php?section=project - 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)