Microcontroller programmers and debuggers
Sparkfun microSD Sniffer can be connected to any microcontroller to detect the signals being passed
AVR USB Programmer is used to program AVR microcontroller using the USB.
Bus Pirate Cable is designed for the Bus Pirate as an interface cable.
SPI Shortcut helps you to control SPI pins over an USB interface.
Pocket AVR Programmer is an affordable low cost programmer.
FTDI Basic Breakout - 3.3V is an easy to use Breakout board which is best suitable for serial applications.