My alternative fantasy game to D&D is Barbarians of Lemuria. It's a lot simpler and breaks things down to really just the essentials. You got your four attributes Strength, Agility, Mind, and Appeal, your four combat skills Melee, Ranged, Brawl, and Defense, and pick four professions, like thief, warrior, sage, healer, peasant, and so on. All start at a rank of 0 and you get four points to distribute in each of the three categories. That's pretty much it and you're ready to go.
For science fiction, my game to go is Star Wars Saga Edition. Even if the game isn't actually Star Wars. I really am not a fan of the d20 system, but Saga is by far the best adaptation of it, bar none.