Open Workbench Logic Sniffer
From Hamsterworks Wiki!
The OWLS is a small FPGA based USB Logic Analyser.
I got mine direct from Seeed Studios (http://www.seeedstudio.com/) for US$62 including two cables. It has proven to be invaluable.
Once up and running the Java client software is really slick.
It is build around the Papilio One FPGA board from http://www.gadgetfactory.net. I'm yet to play with downloading my own designs to the FPGA - the board is much to valuable for me!