ive seen a USB one built...thats probably no use to u though..

The latest range of PIC chips contain a USB stack and its very easy to connect a Ps/2 keyboard to them and log to a serial eeprom as I saw. You could also grab USB with a little bit of thinking

I once tried to modify a USB hub so that I could sniff all the traffic for a certain device that I was reverse engineering to create a linux driver for. Check out USB snoppy or something like that its not a hardware solution and its flawed in may respects. USB protocol analysers do a similar job but in hardware

i2c