Click on the image above for the PDF schematic for a Microchip PIC 18F2550 based USB demo board, with a TTL serial port (also in .ai format).
Or, since I'm reusing their design and also have been using one of their cheap PICKit2 Kit board, I'll mention that you can also buy a more elaborate ready to assemble version of the above demo board from fun4diy.com, that doubles as a PICKit2. Note however that if you use that PICKit2 "Kit" and want serial I/O, you'll have to modify the board a little (but you can use the unused U4 pins for that).
If you're testing USB, such a demo board is very convenient to have, and most of the PIC 18F4550 firmwares you'll see can be easily ported to 16F2550. I'll try to post more about that when I get a chance.