I have recently begun to do code challenges from TopCoder.
I first heard of TopCoder from John Sonmez here: So You Want to Become a Better Programmer? (TopCoder).
I started by doing the archived problems here. The problems are sorted by the percent of competitors who were able to solve them–basically in order of difficulty.
The questions can be fun but a few of them are impossible for me to understand and the interface requires a very large monitor for me to see everything. But overall it has benefited me because I get to attempt to understand a requirement, pass tests, then look at successful and cleanly written code from past competitors.