[zbanks]

I've also spent some time on http://hacker.org

Rather than focus on a specific "answer," you instead write algorithms to solve various flash games based on NP-complete/hard problems. Its interesting since there's always ways to improve. You can start out with a brute-force solver, but as the levels scale your program won't.

They also recently added a "challenges" section, which is more about specific coding tasks. Those are also pretty fun & scale well.

And yes, despite the domain name & poor design, it's actually a pretty serious site. The same guy also did http://goproblems.com/ , which is cool if you're into Go (the board game, not the lang).