Sid Meier's Civilisation IV (plus all Expansions) - awesome game, made even more awesome by the community's modification and customisation work.
X-Wing Alliance - LOL, ok, not much of a "simulator", considering we aren't exactly hooning around space yet, but it's a flight-sim of sorts, and the sheer amount of craft to fly, gaming options, surprisingly solid graphics considering the game's age, and all played to a sweeeeeeet John Williams Soundtrack? Out-standing!
SimCity - Whats more fun than creating a sprawling society, catering for it's every whim, then trashing it with an invading monster? Nothin, that's what :lol:
Gunship 2000 - Old old offering by Microprose; Kicked much ass ;-)
Mechwarrior - played for many many hours back in high school. Again, not really a true simulator, since we're not poking around in large mechanised behemoths, but still loosely fitting into the genre...