Saleae Logic Analyzer
A logic analyzer is an essential tool for debugging communications problems between chips in an embedded system. We reach for our trusty Saleae Logic USB-based analyzer far more often than our mixed-signal oscilloscope for this task. It's portable, has a very polished software interface that's convenient to use, and can capture data for a long period of time, which is key when tracking down intermittent issues. Having the data on a computer where it can be easily exported for further analysis is also handy.