Preferably Android apps available through F-Droid, but feel free to recommend iOS apps, too!

On iOS, I really like Sky Guide—I don't think it's open source, but it has the right features for the casual star gazer like myself, specifically the ability to point my phone at an object (i.e., "what's that object up there") and/or search by name ("where's Jupiter right now"). After my transition to Android, I've found Sky Map, which has good feature parity with Sky Guide.

Any other open source apps like Sky Map that I haven't found yet?