Where can I find programming challenges?

I was gonna post this as a comment to Sebastian, but it got very long.

I have written some comments on all the different pages with algorithmic challenges below. All the pages have problems of varying difficulty from extremely easy to extremely hard, so just pick your favorite and go :)

USACO is america's training page for the IOI (international high-school comp.) it introduces the different paradigms (DP, Greedy, etc) and algorithms (max flow, shortest paths, etc.) very well with some descriptions and then some tasks in that subject. It is very good for people who are new to these kinds of challenges, or people who just want to learn and get introduced to algorithms.

SPOJ, Timus and UVa OJ are all problem archives from different contests and just users uploading problems. They are especially good if you want to have tasks in specific areas (like Dynamic programming, geometry or "mathematical" problems). Also contests are held once in a while here.

TopCoder is the leading site in programming challenges. Contests are held (almost) weekly and with cash prizes (but only if you are #1, which is extremely hard). The problems are very nice and there is a huge database of problems as well. The contest system here also supports a challenge-phase, where you can write test-cases for other people's answers and watch them fail evil laugh. TopCoder also has other competitions like component design, UI design, and much more. It is also a huge community of programmers writing tutorials and much more. I have had much use of topcoders algorithm tutorials.

Codeforces is an "up-and-coming" russian site with competitions evvery 5 days and nice problems. Like topcoder's algorithm part, it is split in two divisions where some problems overlap (usually the hardest div. 2 problem is the middle div. 1 problem). The problems are nice, but some times poorly stated. The problem writers also change from competition to competition, so some contests are more original than others. The community is nice although mostly russian-speaking.

Google code jam is a huge competition held every year. It is much like all the other algorithm competitions. I have not taken part in this myself, so I don't have much to say. It is split in several heats of different difficulties, so even novice coders can participate in the early rounds and learn a lot :)

Interviewstreet differs from the others by being focused on recruiting "hackers" for companies. It holds competitions or "codesprints" once in a while (2 so far, but it's a young site), where competitors get chances to apply to big companies (like facebook, amazon, microsoft, etc.). It also has different types of problems posed by companies, which could be interesting to some. It also gives some insight in what kind of challenges different companies might give you when applying for a job.

http://www.usaco.org/

http://www.interviewstreet.com/recruit/challenges/

http://codeforces.com/

Machine learning: http://www.kdd.org/kddcup/index.php (and the one for 2012 will be posted here in february: http://www.kdd.org/kdd2012/kddcup.shtml )

AI: http://aichallenge.org/

Programming in general, but should be extra fun for functional programmers: http://en.wikipedia.org/wiki/ICFP_Programming_Contest#External_links

Combinatorics: http://projecteuler.net/

Web exploits: http://google-gruyere.appspot.com/

http://www.hackthissite.org/