I've uploaded some ring tones obtained from various MIDI files I found around the web. (I have no idea where though, these are fairly old.)
So point your web-enabled phone to http://www.xenoveritas.org/wap/ and enjoy the custom ring tones!
Update 2007: The mobile site has been updated to use XHTML (so older phones won't work) and moved to http://mobile.xenoveritas.org/.