The GoodFET is an open-source JTAG adapter, loosely based upon the TI MSP430 FET UIF and EZ430U boards, as described in their documentation. It has expanded to be useful as a microcontroller multitool, with 'clients' for programming/debugging MSP430 JTAG, Nordic RF, Chipcon 8051/SPI, AVR, PIC, etc.
A handy tool for advanced microcontroller work, and especially for MSP430 fans who want their own JTAG debugger for everyday use. Comes as a fully assembled and tested board. You may also want to pick up some female/male or female/female jumper cables to connect from the 14 pin header to your dev board.
For a lot more information including all schematics, layouts and firmware, check out the goodfet website
GoodFET v42 by Travis Goodspeed (3:40)