A couple of challenges

The Google code-jam. Algorithmic, ACM-like programming challenge (don't forget about UVa if you want to get some warmup).

A "find the vulnerability" type contest from Immunity.

A Javascript compression contest - this is not an "official" contest (in the sense that there are no prizes and no very strict rules), but more of a "one-upmanship". Still interesting though.

Update: I almost missed this one: Network Forensics Puzzle Contest.