2400 is a fantastic sandbox of ideas and tools that I love to use by itself, or with other games. Sometimes I just use a prompt or a mechanic and run with it. It's perfect for solo games and for larger world building. My only complaint is that it's constantly being updated so often and it can be hard to make sure what I've downloaded is current.

Have been looking for a simple universal sci-fi RPG for awhile, this one is my go to. It's well formatted and easy to pick-up-and-play, offers tons of different subgenres, and is easy to hack! Do you see that robot wrestling a lion, that could be you, that should be you! Play these! Would love to see them someday collected into an A4 hardcover to sit next to my copies of OSE

An excellent set of rules-lite TTRPG campaign settings, mostly science fiction in various flavors. Big focus on interesting characters and tools the GM can use to generate NPCs and missions. The DM can easily mix together zines for their campaign, though it's recommended to have the players stick to one packet for character creation.