MW:LL is a Total Conversion (TC) for the game Crysis Wars (available over Steam at ~7.50 USD). The gameplay should be recognizable to everyone who has played one of the Mechwarrior games before; it's essentially a first-person action-simulation of the BattleTech universe.
What sets this game apart from, say, MechWarrior 4, is the possibility of piloting not only different kind of BattleMechs, but also Tanks, Hovercraft, APCs, Aerospace Fighters, VTOL or even playing as BattleArmor! Then, of course, there's the highly advanced graphics provided by the CryEngine, allowing for detailed models and maps.
MW:LL has been in development for more than two years and is currently still in Beta, but very advanced and stable to play, with Users online at all time. The learning curve can be pretty steep for beginners, but there's a wealth of information available on the official forum and wiki, and of course players ingame will be happy to help you out with any questions you might have.