Here is a list of websites, which are sets of algorithmical tasks or online contests:
http://www.topcoder.com
http://www.spoj.pl
http://icpcres.ecs.baylor.edu/onlinejudge/
http://acm.timus.ru
http://acm.mipt.ru/judge
http://www.uwp.edu/sws/usaco/
http://acm.sgu.ru/
http://projecteuler.net/