The iNaturalist API is an absolute gem. It doesn't require authentication for read-only operations and it has open CORS headers which means it's amazing for demos and tutorials.

My partner and I built this website with it a few years ago: https://www.owlsnearme.com/

(I realize this is a bit on-brand for me but I also use it to track pelicans https://tools.simonwillison.net/species-observation-map#%7B%... )

I also love the Seek app that they provide (maybe this overlaps with the linked app in functionality?). As someone who's grown fonder of Nature in general over the last decade but who has little actual knowledge of the regional flora and fauna, it's a great way to engage with the plants and little bugs in my garden (or others' while on walks and such).

Fun to travel and "pokemon" some new local stuff too.