List of Problemsets

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.

Topcoder - http://www.topcoder.com - Weekly programming competitions from algorithms to components to marathons.

USACO Contest - http://ace.delos.com/contestgate

Project Euler - http://mathschallenge.net/index.php?section=project - Project Euler consists challenging mathematical or computer science problems.

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

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

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

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

UPC2001 Internet Contest - http://contest.psu.ru/Inet

Comenius University, IPSC - http://ipsc.ksp.sk

Moscow SU Virtual Contests - http://acm.msu.ru/virtual

SPb IFMO Training Sessions - http://neerc.ifmo.ru/trains/