New kid in the block of Sci-Fi rules, and my go-to system for casual Science Fiction Roleplay.
Born from Stellagama Cepheus Quantum one-pager, these rules are fast but not trivial. They utilize a dice-pool Xd6 mechanics, simpler than Cepheus' modifier arithmetic for conventional 2d6. The rules include:
Fast character creation - even faster with my FTL:Nomad Generators! No lifepath mechanics though, which might be a miss when referring to a Traveller-like game. But again, chargen in Traveller occupies a whole session of game time, a luxury when one has resources only for a quick game.
Psionics
Computer Hacking
Robots and Xenofauna construction rules
Vehicle and Spaceships combat that are fast (and somewhat unified)
Detailed but also fast space ship construction mechanics. These rules were expanded one of the next supplements.
Lots of tables for random encounters.
The A5-format book became my handy traveller companion.