A whopping 201 fandoms made the cut! There should be plenty for both writers and artists to choose from.
How the art/stories requesting/offering works: When you go to sign up, there will be a space for "Optional Tags". Here is where you will insert "fanfiction" if you want a story, and "fanart" if you want artwork. If you want either one, then put both tags. You have that option for both your requests and your offers.
The sign up form can be found HERE.
And I've made a big list of all the eligible fandoms. Fandoms 2015