@kirschner @fdroid for me, probably the most useful (after maps which has already been covered) is mail. I would recommend https://k9mail.app/.
I use it for work daily and it gets updated regularly.
@kirschner @fdroid I’ll try harder then :)
#Owncloud client https://owncloud.com/
We use it to connect to our own server for storing receipts, plane tickets, company documentation etc. It’s great to keep things synced between devices.
A similar thing is #syncthing, which is cross platform although I don’t have a use case for it at the moment, sadly. https://syncthing.net