One I like is Wechall. It's sort of an index of other challenge sites (including a nifty API that can track your progress through some of their indexed sites),

One I like is Wechall. It's sort of an index of other challenge sites (including a nifty API that can track your progress through some of their indexed sites), although they have a few of their own challenges as well. The sites it indexes tend to be geared towards security topics like web security and cryptography, though there are some more general programming challenges as well.