FASA published such games as
BattleTech,
Shadowrun,
Earthdawn,
Crimson Skies, and
Vor. Their games didn't have a core underlying set of
rules as other
publishers sometimes do (such as
White Wolf Game Studios'
Storyteller system, or
Steve Jackson Games'
GURPS system.)
Sadly, FASA is no more. They've closed their doors. Earthdawn was sold to Living Room Games a couple years back, and Shadowrun is now owned by WizKids LLC. Let's hope good things continue to come out of those game lines.