Speech to text, Voice typing is cool.
F-droid lists opensource apps that support Whisper and Vosk models.
Futo voice is not opensource, but is source available.
It’s good for English. For my mother tongue, Malayalam, the support seems to be underdeveloped currently.
Handy seems to be good for Windows, but have not used it much.

Then for Translations: https://f-droid.org/packages/dev.davidv.translator/
Using Mozilla’s translation Models. A good enough offline and Open source alternative to GTranslate.

Are there other cool uses of offline ML/AI that you know?