I think I wrote about the bus pirate some time ago.
A logic analyzer is a device that lets you watch digital signals in your electronics project. You can watch them real-time or log the data for later perusal. Unlike an oscilloscope, its not good for measuring analog signals – but also unlike an oscilloscope, you can track 8 signals at time! So its a good complementary tool. This logic analyzer plugs into a computer and has easy to use, cross-platform software. This makes it small, portable and inexpensive. If you ever have to to debug SPI, i2c, serial, CAN, 1-wire, Manchester, biphase or other digital protocols, this tool is essential!