PlanetsCentral

This option is on the game page, where it is for every game: either click your race name and choose "resign", or use the "Settings" button and find a "Resign" option at the end.

If you're now telling me that this doesn't quite match current UX standards, you're absolutely right.

Wow, I would never have found that, but good to know the option is there, should I need it. And maybe this thread might help someone else find it too, until you find that magic stone.