At the moment I have 5x RTL-SDR dongles streaming into a box on different bands (137, 433, 912, 978, and 1090 MHz). They pick up all kinds of things, from local weather stations to the water meter for my condo to ADS-B/UAT transmissions from local aircraft. I even see the tire pressure monitor signals from cars that drive by (some of you guys need to add air!).

Mostly it's just fun. I like peeking at all the ambient transmissions in the area.

A while back I was more deeply involved in the SDR space and even reverse-engineered some pager signals. At least a few years ago, pagers were still widely used in hospitals (totally in plaintext, by the way).