Some of this is subjective, just because I see people mentioning a bunch of games that I would consider spinoffs. Doesn't really matter though. Here are my picks:
Astroboy: The Omega Factor
Untitled Goose Game
That's mostly it. Even the games I could mention that are obscure to most would still be a sequel or spinoff (or later had a sequel or spinoff). The video game industry is full of them.