In my spare time I enjoy solving programming puzzles, often algorithmic or mathematical in nature. Here is a list of some of my favorite online judges.
Codeforces
Codeforces probably has the most active competitive programming community. I occasionally participate in their contests. See my profile.
Codechef
CodeChef is another online judge with nice algorithmic problems and regular contests. See my profile.
Hackerearth
Conduct automated online programming tests for developers. HackerEarth provides enterprise software solutions that help organizations in their talent assessment and innovation management endeavours. See my profile.