You can make a variety of simple forms of test equipment with an Arduino, and although it won't replace professional or purpose-built equipment you can get away with some useful tools. One such tool is the single-channel logic sniffer by Arduino forum member "Riva" who uses a simple digital input pin to measure logic signals and pass the data to a processing application on a PC. This clearly shows the logic levels and can be useful for debugging various digital outputs from other devices, buses and so on. Below is an example of the display output:
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