Its here. The famous obdev AVR USB firmware.
**broken link removed**
With it, basically any AVR running at 12MHz can communicate with the PC via USB1.1. Even a Tiny2313 with 2K Flash can do a USB<=>RS232 translation, using the CDC class of the USB standard. A 8-pin Tiny45 works too.
AVR - CDC
There are also many more projects using the obdev firmware USB engine in the first link. Similarly using the same USB engine, I have just built a project on an Mega8 AVRUSB_IR infrared capture mentioned in another thread.