USBWiz can be use in SPI/I2C and with simple USART communication. Look at their FAQ

http://www.ghielectronics.com/faq.php?id=5

Pricing is fair enough ~20USD

You'll need to play a little bit with it... and read the whole set of PDF on their website... even if they're a bit badly explained (my personal opinion... i saw way better before... but worst as well) . The user manual is only 53 pages long.
http://www.ghielectronics.com/downlo...z%20Manual.pdf