Anon, i have found it very helpful – though time-consuming – to look up each individual rotorcraft to find out who owns and operates it. State Police, traffic, TV news, firefighting, medevac, big banks (yes they do run choppers, I have seen JP Morgan Chase's chopper on radar), sight-seeing, military training, military exercise, etc. Just circling the number of choppers is a little bit interesting, but a little bit deeper digging is more useful.
ThankQ for watching the choppers though. That's a really big job and tedious.