Crysis and Crysis: Warhead are a couple of really good shooters from the mid-'00s that helped cement Crytek's reputation, at least briefly, as one of the world's top-tier FPS studios. They've been available for some time on Steam and Origin, but now they're on GOG as well, which is good for two reasons: There's no DRM, and they're on sale.
Crysis zips players up in a sexy, skin-tight "nanosuit" that greatly enhances its wearer's capabilities, and sends them off to clean up a conflict with the North Koreans that eventually turns into an invasion of aliens from beyond the galactic rim. Something like that, anyway. The plot didn't make a lot of sense, to be honest, but punching guys into orbit like a battery-powered Superman never got tired, and the gunplay was excellent too. Crysis: Warhead is basically more of the same, except Crytek realized that Psycho, an NPC from the first game, was a lot more interesting that the original hero character, and so they put him in charge.
GOG has also added the Second World War FPS Medal of Honor: Pacific Assault to its lineup. It probably hasn't aged as well as the cutting-edge (at the time) Crysis games, and I think suffered somewhat from the effects of franchise fatigue when it was new, too. Even so, it's a very good shooter in its own right, especially if WW2 is your thing.
All three games are on sale for $8 until November 3. GOG has other EA games on sale as well, including Dragon Age: Origins ($8), the original Mirror's Edge ($10), and Dead Space ($8), until October 31.