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 We have many other translations, but they may be out of date.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.
Since Mumble only supports a very limited set of administrative capabilities from the client itself, we are in need of additional Management solutions for the Murmur server. These could be written in any language supported by our the Ice based RPC Interface (there is also DBus but it does not receive updates anymore and might get removed completely in the future). You could start your own project or link up with an already existing one (an incomplete list of existing Interfaces can be found here. To get an idea about how interaction with murmur might look like you can take a look at the example scripts in out git repositories script folder.
We currently ship with an dbus authenticator that works with phpBB3. It would be a real bonus if we had more authenticators for our current RPC protocol of choice, Ice. These could be for other bb software, blogs, anything you can think of. Sample authenticators can be found in our repositories script folder.
We are always looking for helping hands willing to contribute to mumble. You'll need a good understanding of C++ and previous knowledge in Qt is definitely helpfull. The best way to get into mumble development is to link up with us on IRC (#mumble on freenode), there is always small stuff to do to get you started.