Calibre Companion, on android. For years it was the best way to sync books between my calibre install on my computer and my phone, even after the developer abandoned it. Then a while back, the dev released a single update that broke the app’s integration with reader apps, then disappeared again. I rolled that back extremely quickly, by grabbing an older version from my old phone.

Sluggy Freelance is a classic; it’s one of the oldest, longest running ones out there.