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).

It is cool to do. But there are professional uses for it, particularly like finding interference or noise. You can ask things like: "How crowded is my 2.4 GHz spectrum?"

The RTL SDR is the gateway drug into SDR's. The bandwidth is limited, so don't expect to be capturing HDTV Streams. But it's certainly possible to listen to AM/FM, Ham Bands, VHF/UHF signals, etc.

If you don't want to buy an RTL-SDR, you can search KiwiSDR which is a streaming SDR for frequencies under 30MHz. You can also connect to KiwiSDR's worldwide.

Then if you get the itch, you can go get your a HackRF, a ham license, or whatever else scratches it.

I've used it to identify transmissions in my neighborhood in the public (ISM) band, listen to various radio broadcasts outside the normal AM/FM, and best of all, track all the planes in my area (ADS-B).

Satdump group uses them receiving all kind of weather satellites. They are mastering S-/X-Band stuff for ages already.

Ham radio operators looking forward using them for 13cm and 5cm band stuff.