The BusPirate v3 http://dangerousprototypes.com/docs/Bus_Pirate is a universal bus interface that talks to electronics from a PC serial terminal. Get to know a chip without writing code. Eliminates a ton of early prototyping effort with new or unknown chips. 

Protocols

1-Wire, I2C, SPI, JTAG, asynchronous serial (UART), MIDI, PC keyboard, HD44780 LCDs, and generic 2- and 3-wire libraries for custom protocols.

Features

    USB interface, USB powered
    5volt tolerant pins
    0-6volt measurement probe
    1Hz-40MHz frequency measurement
    1kHz – 4MHz pulse-width modulator, frequency generator
    On-board multi-voltage pull-up resistors
    On-board 3.3volt and 5volt power supplies with software reset
    Macros for common operations
    Bus traffic sniffers (SPI, I2C)
    Transparent USB->serial bridge mode
    10Hz-1MHz low-speed logic analyzer
    Custom support in AVRDUDE http://savannah.nongnu.org/projects/avrdude, Flashrom http://www.flashrom.org/Flashrom, OpenOCD http://openocd.berlios.de/web/
    AVR STK500 v2 programmer clone
    Scriptable from Perl, Python, etc.
    A bootloader for easy USB firmware updates
    Open source (CC 0)

Support

Bus Pirate manual http://dangerousprototypes.com/docs/Bus_Pirate
v3.5 hardware overview http://dangerousprototypes.com/docs/Bus_Pirate_v3.5
Chip demonstrations http://dangerousprototypes.com/docs/Bus_Pirate#Chip_demonstrations
Latest firmware http://code.google.com/p/the-bus-pirate/
Bus Pirate forum http://dangerousprototypes.com/forum/viewforum.php?f=4