online judge

UVa http://uva.onlinejudge.org/

UVa-live (live-archive) http://icpcarchive.ecs.baylor.edu/ http://livearchive.onlinejudge.org/

SPOJ http://www.spoj.pl/

Topcoder http://www.topcoder.com/tc

POJ http://poj.org/

Project Euler http://projecteuler.net/

IPSC http://ipsc.ksp.sk/

Codeforces http://codeforces.com/

USACO competitions http://ace.delos.com/contestgate

USACO training http://ace.delos.com/usacogate

codechef http://www.codechef.com/

Timus http://acm.timus.ru/