I've always enjoyed problem solving and have actively participated in online forums such as Sun's Java Developer Forums, Java Ranch and Google Groups. They allow you to expand your knowledge, share what you know with the online community and even help you establish yourself as a bit of a guru (if you're really good!).
I especially like Sun's Forums because they allow you to earn Duke Dollars based on how good your answers are. (I am currently worth 139 duke dollars!)
Recently, I stumbled across Yahoo! Answers which is a place to ask questions and get real answers from real people. It’s fun and educational because you can ask questions on any topic—from the serious to the not so serious. And when you know what's up, you get to help people out by answering their questions. You earn 2 points for answering a question and 10 if your answer is selected as the best answer! This has really got me hooked. I mostly answer the Mathematics and Computing questions but I also like taking part in the more general categories like Society and Culture.
Here is my little answers badge which shows all my questions and answers: