RadioReference Complete Frequency Database

A related resource specific to amateur radio and GMRS repeaters is https://www.repeaterbook.com/. Very useful to find repeaters near you, if you're looking for them. You can find them in RadioReference too, but they tend to be a little more out of date.

This site is indispensable if you're into frequency scanners. You still need to curate your lists, because there is a lot of dead and useless channels, but it sure beats the time it takes for one to manually dig up and compile frequency lists yourself.