This is a very affordable 24Mhz 8 Channel USB Logic analyzer. This is a hobbyist level analyzer, ideal for testing your Arduino or other hobbyist microcontroller's communications.

A logic analyzer is a must have tool for any serious electronics hobbyist. With a logic analyzer it is possible to see a visual representation of the zeros and ones that are flashing by between two components. A logic analyzer is different from an oscilloscope because the analyzer allows you to capture the data on a PC for a more detailed analysis of the data which is being transmitted and received.

This logic analyzer supports 10 different communications standards (see Specifications below). with 8 channels it will allow you to monitor upto 8 differnet wires. So on an Arduino you could monitor the Serial port (tx/Rx pins), the I2C port (pins A4, A5), and the SPI port (pins 10, 11, 12, and 13) all at the same time.

Specification