MortRing

A pretty simple app to assign ringtones to contacts.
No bloated contact manager, no bells and whistles, just a simple list of contacts and assigned ringtones. Tap a contact to assign a different ringtone. That's all.
(Uses Android's ringtone selection. No download, no free file selection.)

It's still in pretty early development state, and was mostly made for personal use. It seems pretty stable to me, but there might be dragons... ;-)