Online Programming Contests
(in no particular order)
[*][url=http://www.spoj.pl/problems/classical/]Sphere Online Judge[/url]
Supports more than 30 different programming languages and a growing problem-set of about 3000 tasks for practice.
A “series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve”.
[*][url=http://icpcres.ecs.baylor.edu/onlinejudge/]UVa Online Judge[/url]
More than 200 contests and more than 2000 algorithmic programming problems.
Not only algorithmical, but also architectural and design contests.
If you still haven’t got enough, check out the [url=http://www.google.com/Top/Computers/Programming/Contests/]Google Directory of Programming Contests[/url].