List of games that are free/single-player, worth it, will update occasionally.
Star Control 2: The Ur-Quan Masters
Further info: Star Control 1 is also free but it's not worth it, it's pretty much a war sim, with little story that's explained in StarCon 2.
Iji or any game by Daniel Remar. http://www.remar.se/daniel/games.php
Further info: Iji is like the Undertale of Shooters, but like, 10 years older.
Further info: Does have paid versions, but DON'T get the 3D remake on the 3DS. Is bad.