Waterfalls: not the river kind, but the audio kind. Also known as sonograms, those are 2D+1 representations of the spectrum of a signal: the X axis usually represents the frequency, the Y axis the time (or the other way around), and the power of each frequency is drawn on a color gradient.
In the ham radio world, waterfalls are a great way to visualise part of the frequency spectrum. Depending on the type of radio you have – SDR or not, really – , you can either display a fairly wide part of the spectrum, or just a few kilohertz. But in any case, this visualisation will give you a new way of understanding what is going on around you, RF-wise.