Released way back in 2002, Mechwarrior 4: Mercenaries is my favorite in the giant- walking-robot-shooter genre. I played the game's single-player campaign easily a half-dozen times, enjoying the campaign missions and the ability to salvage mechs and use those parts on your own rigs.
Now it's back, and it's free. The brilliant team at Mektek have lobbied Microsoft Game Studios to give up the code and release it free of charge. You can get it here. In addition to the game's stock missions and mechs, the Mektek team's additional mech packs are part of the distribution, bringing the total equipment available to astronomical levels.
If you've never played the game, now is as good a time as any. The jury's still out if we'll see the promising MW5 reboot, but at least we have MW4:Mercs to tide us over.