Hard Reset is a fun, cyberpunk themed shooter. Weapons can be upgraded, have multiple fire modes, the enemies are varied and the gameplay is polished with solid gunplay. It's old-school in its approach and best of all, because it's a PC game it doesn't have any bullshit from having been a console port.
The STALKER games are also really good. Combining some exploration of "the zone", the radioactive area around Chernobyl, survival and fun sneaking/shooting. Get the "Complete" mods to flesh the games out, fix bugs and add some extra polish.
Painkiller games are old school shooters also, but where Hard Reset is robots and sci fi, Painkiller is demons and hellfire. You have really cool weapons to play with and no dicking about with anything other than the killing of the damned and the dead.
Crysis and Crysis Warhead are good PC shooters, the only games in the series worth playing since shortly after they devolved from top-notch PC shooters to console MMSs. You have a nano suit allowing you to run fast, jump high, turn invisible or absorb damage and multiple approaches to tackling objectives. Avoid 2 and 3 at all costs, they're so awful you'll regret buying them.
Rage from id was alright. Built on their newest engine, it looks very pretty. It's very much like Borderlands but with a lot less faff and downtime between actual shooting.
Bulletstorm is pure shooter. The protagonist is Oghren from DA:O and you basically get lots of funky weapons and get "scored" for zany gunplay. Shooting a dude gets you 10 points. Shooting him in half and impaling his torso on a cactus gets you 1000. Yuu get energy whips, bolas, blasters, rifles and more. This is a puerile, vulgar, juvenile game in all ways but the gunplay is good. Made by the same folks who made Painkiller.
Singularity is basically BioShock but with a "time travel" twist. You get temporal related abilities/plasmids which you level up with Adam and it's a lot of fun, at least once. Favourite thing is freezing enemies with liquid nitrogen.
Other honourable mentions go to FEAR, The Darkness I/II, Star Wars: Jedi Academy, Dead Space 2.