The XMMS2 Project has come a long way since 2003, but there are still a lot of rough edges around. If you'd like to help make XMMS2 a more interesting project, then check out the following items. We always welcome more manpower that can help our late project later.
Programming and similar
Low hanging fruit
Some easy tasks that can be well suited for familiarizing with the XMMS2 codebase (and patch nazis) can be found in our bugtracker. 1638: Low Hanging Fruit.
Long term wishlist
We have a whole bunch of feature requests (and possibly bugs) filed away on our Long term wishlist that we haven't gotten around to implementing or fixing yet. You could poke around there and see if there's anything that interests you.
Clients without maintainers
The Clientlist is full of clients in every programming language and with every gui toolkit you can imagine. Why not adopt an abandoned, but maybe promising client? Most of them are open source and most developers would be happy to see someone continue what they started.
While XMMS2 steadily moves forward, the Wiki is not always treated with the same respect. Pages may be forgotten for years and proudly announce XMMS2 0.4DrKosmos to be the newest, shiniest release you could get. Those obviously outdated pages should be corrected, or collected on the XMMS2:Wikitodo page.
Testing and Bugreports
You have found a bug in XMMS2 or maybe a client? Great! Report it on the Bug Tracker or on the Mailing List. Alternatively, see if you can find a bug to reproduce and further describe on the bug tracker.