Andrew, Souceforge.Net is a great site for open source projects...actually more than 50000 of them. The ubiquitous jBoss (atleast in the Servlet Engine discussions) is a product from this initiative. There is a link asking for developers to help them with projects. Then there is Apache(www.apache.org)..but getting in a project requires some effort.. Goodluck! Thilan
All of the Jakarta projects are quite interesting. Latka (automating acceptance testing for web apps) is a newer one that could use some help. I've recently taken an interest in Prevayler, which is a brilliant open source project for robust, persistent business objects without a relational database.