Mumble is a great program, but that is no reason to stop polishing it. There are numerous areas where the development team needs help, and if you feel like contributing, then any of the below is for you.
The Wiki documents are more or less up to date. It would be very helpfull if someone went through them all and made them completely up to date.
Someone that is not a development team member needs to maintain the article at Wikipedia. At the moment it's missing references; try to find some. (This requires more work than just googling).
Mumble is currently actively translated in German and Spanish. We have many other translations, but they may be out of date. If you wish to do a translation, get in contact with the dev team through the forums.
For a tutorial, see Language Translation.
Get the word out. This means more than writing about it on your blog ;) There are many providers of free murmur servers; coordinate with one of them and start promoting Mumble as the voicechat of choice for your favorite game.
Create a new skin for Mumble. Mumble supports extensive skinning, but so far this feature hasn't been used much.
For a tutorial, see Skinning.
Murmur needs a management suite. Somthing much easier to use and more sophisticated than the silly examples we bundle.
We currently ship with an authenticator that works with phpBB3. It would be a real bonus if we had more authenticators.
PHP D-Bus bindings
To use the DBus interface in webpages, it would be very beneficial if there were PHP bindings for DBus. This will be a bit of a job though, especially if you're going to support full introspection.