I'm not sure if they're popular or not but I didn't see them mentioned in this thread and I rely on these fantastic open source apps daily:

1. AntennaPod -- podcast app
2. Orgzly -- Todo lists / reminders / notes via org files. Can be synced via syncthing.
3. Aves -- gallery app
4. KeePassDX -- keepass client
5. Prognoza -- weather app
6. Olauncher -- minimalist launcher

Honourable mention: Harmonic -- hacker News app. Not "essential" by any means but it's quite good.

Edit: added prognoza and olauncher

Magic earth is not open source, if that matters to you.

Voice is wonderful https://github.com/PaulWoitaschek/Voice

In addition to GPay contactless, Android Auto will also not work on GrapheneOS.

Huh? It has a dark mode in Settings->Theme.

As for ad-blocking, I have found that system-wide blocking works great as suggested in the docs https://grapheneos.org/faq#ad-blocking. YMMV...

Cromite indeed looks great.

This may not be helpful, but if you're willing to flash GrapheneOS on a pixel, Vanadium is wonderful.

Bromite has been abandoned. https://github.com/bromite/bromite/issues/2641

Ungoogled chromium binaries are built by anyone so don't get them from there. https://github.com/ungoogled-software/ungoogled-chromium#third-party-binaries