Have you looked at the Amtel AT89C5130?

Otherwise, try putting:

USB 8051 interface

into a search engine (google or whatever).

This is probably one of the easiest routes:

http://www.ftdichip.com/

They have parts that convert usb to uart and provide the system drivers to use with their parts. The next step up, doing your own usb, is an investment but most major microprocessor and microcontroller manufacturers have development kits to help ease the pain. The real show stopper is when you get to having to have drivers and purchase a manufacturer's code. It is in another realm compared to the good old days of serial and RS232.