Since I've been seeing people making posts regarding their interest in helping out with various things, I'm going to make this stickied thread here so that the posts all end up in a place that we're more likely to notice. To reiterate, if you have a useful skill and you want to contribute in some meaningful way, post in this thread.
Ideally, you should tell us the following, assuming we don't already know you:
What are you skilled at? See below for some examples of what we consider useful skills.
Where did you hear about us? From a friend? Phoronix? Mod DB? Something else?
Did you play Tremulous in the past? Totally not a requirement, but nice to know.
What else have you worked on? Other projects? Personal things? Have a portfolio or site?
How do we stay in touch with you? Email? Forum messages? IRC?
Once you've posted about yourself and what you'd like to do, wait to be contacted by someone on the team, and then we'll discuss things further regarding the things you can do. There are many things that can be done, and they aren't just limited to modeling and coding. So, even if you're unsure of whether or not your skill is immediately useful, post in the thread anyway.
Now, here are a few examples of things we'd find useful:
Artwork. Concept art, map textures, promotional pieces, etc.
Models. We're really in need of map props. Futuristic scenery, crates, barrels, weird alien things, etc.
Coding. Take a look at our codebase to see what interests you.
Translation. Do you speak a foreign language? Are you fluent in both that language and English? Fantastic, tell us.
Sounds. Can you produce high-quality sound effects? Aliens roaring? Guns firing?
Wiki. Do you have a solid grasp of how to edit a wiki? It's right here.
Of course, that list isn't all-inclusive, but it should give you a general idea.
edit:
Look here: http://unvanquished.net/contribute