I have been playing with some FTDI FT2232D USB chips that are pretty interesting. They are capable of providing a pair of UARTs on a USB cable and they are also capable of providing several varieties of parallel access. FTDI has downloadable libraries and drivers for UART, SPI and I2C as well as simple parallel, uC bus emulation and a couple of other parallel configurations. I have used them as UARTs and simple parallel access. The drivers work pretty well and the libraries that I have used all work pretty well.
The reason that I am excited about this part is that it brings the easy software access back that I had in the old DOS days before protected driver modes and the old ISA days where an interface board was simple to build. To make it even more acessible, they sell modules that have the part, a crystal, and USB connector on a 40 pin DIP footprint. This module brings back the easy accessibility and fast hacking for simple hardware toys. The modules are $35 from Mouser.
I don't have any connection with FTDI, I am just having fun with their parts and though some other people might also.
Enjoy, BobH