I find the Bode plot to be one of the most useful tools in all of Classical controls.  This is probably no surprise to anyone who works in the field.  Many of its uses are obvious.  However, I always wondered if there were a way to determine stability from the Bode plot.  After giving it some thought I assumed there was no way to determine if the pole had a positive or negative real part.  However, the Google group for controls has a topic on determining stability from the Bode plot.  Here’s the link.