There are SourceForge and Savannah for free software; Wikimedia for free knowledge - but no repository exclusively dedicated to free art.
- music, including tracker music, classical compositions etc.
- sound files (samples)
- drawings, paintings of any kind, clip art ..
- web comics (fully remixable!)
- artistic photography (i.e. no general photo archive - there's Commons for functional photography & Flickr for random pictures of your cat)
- stories (including the vast area of fan fiction)
- videos - could include documentaries for now since there is no better space; should exclude random YouTube style home videos
- game projects (i.e. new game ideas)
- 3D artwork/models
- criterion of freedom - all works must be free works (including source data etc.)
- criterion of effort - is it a work worth preserving? (ratings will help here)
- criterion of function - does the work have artistic merit, or is it a work of knowledge?
On "adult content"
Follow the law where the server is hosted and basic ethics (incitement to hate, exploitation of minors, ..). Resist censorship attempts. Judge erotica on its artistic merits, not on moral grounds.
2-3 decent servers should be enough for the first few months. May be a good idea to disable video, but Theora might limit bogus uploads.
- better discussion support - complete LiquidThreads
- file rating support
- better metadata (no starting requirement)
- media players
- importers (see below)
- automatic conversion of all uploaded files to free formats (e.g. Theora)
- generate & package dumps for BitTorrent distribution
For each of these, one could program specialized bots that authenticate as user X with password Y (to prove copyright & build community), then retrieve all the files from the user's profile:
- http://freesound.iua.upf.edu/ - may be switching to a reasonable license soon
- http://www.comicgenesis.com/ - how many artists could be convinced to use a free license?
- http://openclipart.org/ - yay for PD