I'm a fan for free, rules-lite games, so here are some suggestions:

• Lasers And Feelings - a sci-fi system in which the PCs are crewmen on a small spaceship. Includes an optional system to generate a threat/hook on the spot.

• 44 - A Game Of Automatic Fear - an alternate history/sci-fi RPG in which the PCs play ordinery people whose friends, family or aquitences were replaced by robots as part of a conspiracy. The game has its own system with very specific rules and pacing, but it plays nicely and the premise alone is great.

• Freeform Universal (FU) - A simple, generic system, designed for roleplaying and storytelling rather than combat or complex mechanics. It's overall a great system for newcomers or just GMs who prefer quick and dirty roleplay. It can be seen as somewhat unbaked, though, as combat mechanics are somewhat vague, no progression mechanics and the absence of an HP system, but the system is open enough to homebrew some rules.

• Risus - The "Anything RPG", Risus is a 6-page system designed for comedic campaigns, but can be used otherwise. Characters are created using cliches - archetypes or skills - and like FU, Risus has a lot of freedom in its design. The combat is particularly interesting, as getting hurt means one of your cliches are weakened, giving you a tactical disadvantage. But, like FU, it has too few rules, such as no progression mechanics, and so gaps have to be filled. Good for short, tongue-and-cheek campaigns that focus on RPing.