iNaturalist is another gem. Hackers please support these apps so they don’t turn into the AllTrails and Couchsurfing type rackets that we deserve.
I'm currently building out a tool to aggregate and also allow for submitting back to inaturalist, it's so great!
Merlin is run by the Cornell Lab of Ornithology, so it should (hopefully!) remain good and free, unlike apps by private companies with a profit motive.