This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Sourceforge.net and apache.org are full of projects that are looking for help. Or check out http://java-source.net/. I generally advise to pick a project that you like and use yourself, since that makes it more likely that you'll stay interested in the long term (which I think is the only time frame that makes sense to become involved in a project).