Bus Pirate

The Bus Pirate is an inexpensive and insanely useful debugging tool from Dangerous Prototypes. It was first featured on Hack a Day. It allows you to communicate from any PC or Mac to any component that speaks UART, I2C, or SPI. It even has a Logic Analyzer mode, which is not really powerful though. The documentation can be found here.

	SeeedStudio	SparkFun	Adafruit
Bus Pirate v3	$27.15	$29.95	$30.00
Probe Kit	$4.99		$7.00
Cable		$4.95
Hook with Pigtail		$4.95

It looks like SparkFun's cable is different, but this is not the case. Always plug the cable in, with the brown up (if the USB is on the right). The pin layout is this:

Pin	Color	BP Pins	UART	I2C	SPI	KB	Logic
1	brown	GND
2	red	3.3V
3	orange	5.0V
4	yellow	ADC
5	green	VPU
6	blue	AUX					chan4
7	purple	CLK		SCL	CLK	CLK	chan2
8	gray	MOSI	TX	SDA	MOSI	DATA	chan3
9	white	CS			CS		chan0
0	black	MISO	RX		MISO		chan1