http://livearchive.onlinejudge.org
http://coj.uci.cu/OnlineJudge
http://www.spoj.pl
http://uva.onlinejudge.org
http://acm.timus.ru
http://acm.sgu.ru
http://acm.zju.edu.cn
http://plg.uwaterloo.ca/~acm00
http://cs.stanford.edu/group/acm
http://dwite.ca
https://www.facebook.com/careers/puzzles.php
http://code.google.com/codejam/contests.html
http://projecteuler.net
http://www.codechef.com
http://www.interviewstreet.com
http://www.cs.berkeley.edu/~hilfingr/programming-contest/
The bottom of the page also has links to problem sets from a bunch of other contests.
http://community.topcoder.com/tc
http://codeforces.com/
Both of these sites run a few algorithm competitions every month.
http://livearchive.onlinejudge.org
http://coj.uci.cu/OnlineJudge
http://www.spoj.pl
http://uva.onlinejudge.org
http://acm.timus.ru
http://acm.sgu.ru
http://acm.zju.edu.cn
http://plg.uwaterloo.ca/~acm00
http://cs.stanford.edu/group/acm
http://dwite.ca
https://www.facebook.com/careers/puzzles.php
http://code.google.com/codejam/contests.html
http://projecteuler.net
http://www.codechef.com
http://www.interviewstreet.com