[jmagar]

http://addagram.mytestbench.com/

I too have played with that one, but I used JavaScript... These problems are engaging enough to keep you interested, especially when you are trying out a new language. The challenge is significant, and the optimizations required to solve these help you explore some the deeper depths of a language, when compared to a "hello world!" type of problem.

[thaiyoshi]

These types of puzzles are also fun to tinker with to pass time during a long road trip (provided you're with the right people).