I’ve used iNaturalist for a couple years and think it’s a great tool. Two features that help the app stand out from competitors are 1: The machine learning it uses. Once you take a picture, it can automatically suggest what species you’re looking at. 2: With every photo you upload and tag with location and other metadata, you’re contributing to real science. iNaturalist shares data with scientific data repositories like the Global Biodiversity Information Facility.
The app is a joint initiative between The California Academy of Sciences and the National Geographic Society and works sort of like a Shazam for Nature in that it lets you snap a shot of something you come across and instantly get an answer for what that planet, animal, or bug might be.