Good games on the PC?

a * next to the game means that it has nice graphics and will make full use of your videocard.
a @ next to the game indicates a game that I found to be good but not unique (boring after a bit)
FPS
*Bioshock - FPS with RPG elements. Good story. Nice graphics.
Half-life 1 and *HL2 - Sci-fi FPS. Good story. Good gameplay. Excellent mods made with engines.
HL1 Mods
Science and Industry - Premise is to capture enemy scientists to make more money than your rival company. The more scientists you have, the faster you research technology of your choice to kill the enemy. There are additional ways to make money depending on the map.
Natural Seleciton - FPS/RTS hybrid. Humans vs aliens. Human team has one commander
(RTS) that places structures and the FPS team members build them and kill the aliens. Aliens don't have a commander but they can choose a builder-unit to place structures in first person.

Counter-Strike and *CS:Source - FPS. Basic terrorist vs counter-terrorist shooter. Not a mod anymore since it's sold seperately but based on HL1 engine. Source version HL2 engine.
Day of Defeat and OD:Source - WWII FPS. Same deal as CS and CS:S.
HL2 Mods
ystopia - Sci-fi FPS. Cyberpunk game where Punks are trying to hack/break into Corps stuff to complete objectives. To get into cyberspace to hack you need to find a jack-in point (computer terminal) and so the two "worlds" are well connected. Classes are fairly customizable for different tactics.
*@Pirates Vikings and Knights 2 - Remake of PVK for HL1. Very nice graphics. Basic deathmatch between three sides.

*Left 4 Dead (L4D) - Co-op FPS. Lots of fun with friends.
*Team Fortress 2 (TF2) - Teamplay FPS. Different gamemodes from Team Fortress classic and it plays out quite a bit differently. Cartoony feel.
*@Call of Duty 4 - Modern day FPS
*@Battlefield 2 - Another shooter like call of duty. 2142 is the sci-fi version. Neither BF2 or CoD are unique so I got bored of them.

Other
*@Assassin's Creed - Action game, I guess. Cool graphics. Unique gameplay.
*@Prince of Persia - Platform game, I guess. Cool graphics.
Aquaria - Adventure game. 2D with nice graphics. Cool movement and story.
Armadillo Run - Puzzle game where you make devices to get a ball from one place to another. Realistic physics.
Subspace (a.k.a. Continuum) - Spaceship shooter. Not new, not good graphics.
Escape Velocity Nova - Spaceship RPG. 2D but very fun. If you want something newer try Freelancer.
Portal - puzzle game.

RTS
Metal Fatigue - Older RTS. Gameworld is split into three levels: air, land, and underground. Each level is important to control for different reasons. Regular C&C style RTS except you can make customizable mechs as well. Your mechs are your main fighting unit except they can't fit underground, which is where you have to use tanks.
NetStorm - I wrote a blog about it here. Only RTS I've ever played as seriously as SC because it has large room for perfection.
Dawn of War - RTS where you capture control points for resources. Most squads are customizable and reinforceable. Lots of races.
*@Dawn of War 2 - More tactical version of Dawn of War. Don't have a base to manage and instead you control a few squads of units that have many abilities.
*Company of Heroes - Very similar to Dawn of War except it's WWII instead of sci-fi.
RPG
*The Witcher - Only good new RPG I've played. Quite long.
*@Knights of the Old Republic 2 - Pretty decent. Heard 1 was better but it doesn't have as nice graphics if you care.
*Sacred 2 - an RPG similar to diablo 2. It has much nicer graphics, completely different classes, and a more open world. You can also get mounts and customize your skills.

Hybrids - the first genre mentioned is the major one
*@Fallout 3 - RPG/FPS (or TPS) elements. Open world compared to Bioshock which is linear. Also, more of an RPG than Bioshock.
*@Oblivion - Same thing as Fallout 3 but medieval.
Heroes of Annihilated Empires - Fun RTS/RPG hybrid that's fun to play with friends. Little buggy and imbalanced but it's still very fun.
Savage 2 - FPS(or TPS)/RTS hybrid. It's "free" now and runs on win+linux+mac, so worth checking out. Similar to natural selection, main difference is that it's beast vs humans instead of aliens vs humans.
Mount & Blade - Medieval FPS(or TPS) RPG. Good combination of commanding army and fighting as a soldier.

These are some games that I've found enjoyable. It really depends on the style of game you want to play as each one is unique. The one's I called repetitive I didn't bother beating but they're still fun games. For this reason, I didn't list many RPGs (really only Fallout 3 and Mount & Blade) because I get bored of them as none are unique.