Deus Ex 1 is still very playable.
Both technology-wise and mechanically.
Hitman: Blood Money is the best Hitman game.
A little on the short side though, makes up for it with fair amount of replayability.
Painkiller:Black, Serious Sam FE/SE/3 (+-HD) if you're into classic horde FPS shooters.
May or may not be hard, it's on an individual basis.
(See Also: Shadow Warrior if your system can run it)
Sleeping Dogs is a good Third Person Sandboxy beat em up.
Think GTA with more Kung Fu, and you're a cop instead of a crook.
Shadowrun Returns / Dragonfall are Niche but fun RPGs.
Combat plays similarly to XCom:EU/EW but with RPG dialogue and story.
Wasteland 2 if you *really* like old Fallout 1/2 style games.
More Niche than Shadowrun and harder to pick up and go.
Torchlight 2 if you want a competent Diablo Like.
Seriously, it's literally Diablo 2.
Dead Rising 2: OTR if you can handle a little bit of GFWL.
Smash zombies in fun ways! Rescue people! Combo Weapons! Controls pretty good on M&KB!
Dungeons of Dredmore if you want an accessible Roguelike.
Because Losing can be fun when you do it in style.
Guacamelee if you're into MetroidVania.
Luchadore themed sidescroller with modern sensibilities.