if you're developing a client, maybe you'd like to add it to the clients page. You can also discover and add bits of wisdom to Writing XMMS2 Clients.
If on the other hand you feel like there's already enough half-baked clients out there, why not look at the Clientlist and see what projects could use some contributions or a new maintainer.
if you want to develop a plugin, you should familiarise yourself with the Design of XMMS2.
if you want to submit code that fixes a problem, or adds a feature, you should read up on our guidelines for working with XMMS2 code.
if you want to contribute to this wiki, check out the to-do list and guidelines for the wiki, or just edit what you can. Please note that this wiki does not allow anonymous edits, as we'd like to know who is changing what, and when, so please register and log in.