Man, so many games, so little time.
Titans of Steel:Warring Suns. It's battletech with the serial numbers filed off, freeware, and freakin' awesome in its detail. There's nothing like hitting center torso on a recon mech with an AC 20, causing a reactor meltown, leading to an explosion chain reacting to other big robots. It only happened once, but there are lots of "only happened once" events. After many hundreds of hours of play, I just got back to back AC 20 headshots taking down an otherwise perfect mech.
It is an RPG, by the way, as you recruit pilots (humans, clones, androids, cyborgs), and train their skills and abilities.
Too many others, but that's a game I keep coming back to.