Right now there's a great contest going on called the My360Theme Contest. It allows users to create Xbox 360 Dashboard themes, upload them, and then be voted on by everyone. I've just uploaded the Official Co-Optimus Theme, so everyone should head on over there and vote!
In fact - I highly recommend everyone creates and account and votes for our sister site - Evil Avatar. You can vote for as many themes as you want! And if you want to upload one of your own for your chance at winning a prizes, make sure you affiliate yourself with EvAv.