Young's Modulus is a mech simulator from Austin Meyer, author of the X-Plane flight simulator and Space Combat freeware space simulator. Young's Modulus was payware until short time ago, when it was made freeware.
YM features a war between Terrans and an invading alien race (very original plot, I have to say

), and the player has to fight them with mechs.
2 game modes are available: quick mode and strategic mode.
In quick mode, you choose which mech (or better ATAW, All Terrain Attack Walker) you'll drive, as long as what mechs will be used by your wingmen. Obviously, you can choose from various chassis, and then choose which weapons and equipment use. ATAWs are not as customizable as mechs in MechWarrior, but I suppose I can't complain since it's freeware and of good quality.
In strategic mode, it also doubles as a turn-based strategy game where the player has to conquer other planets and free planets conquered by the aliens. Obviously with resource managment

. When a battle starts, the game switches automatically to a quick-like mode. The strategic side of the game is simplified, but is good.
The only criticism I move to this game is, unlike other games by Austin Meyer, the lack of a built-in "vehicle editor". X-Plane has Plane Maker. Space Combat has a spacecraft builder. Young's Modulus does not have a mech builder. I think this subtracts a lot to this game's longevity...