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