Chirp is a badass piece of software written to modify radio settings and channels from your computer.
Radio Reference is a massive repository of radio frequencies from all over the US. Radio Reference has many local law-enforcement and first-responder frequencies as well as federal, municipal, business, and amateur frequencies. The best part is paid members can modify their channels online and import directly into Chirp to download to your radio.
Repeater Book is a comprehensive directory of amateur and GMRS repeaters. Note: a GMRS or HAM call sign is required to obtain a Repeater Book account.
My GMRS is a national directory of listed GMRS repeaters.