Let's not forget Steam games. Civilization V is worth every penny. I've put more than 350 hours into that game since launch. It's a turn based strategy where you create your own civilization and compete against others (eg, throwing the world into a nuclear war or coming out as the culturally superior civilization). Fantastic replay potential.
Portal and Portal 2 are also amazing games. They're puzzle games. The second is hilarious.
Half Life 2 has aged a bit. At its release date, it was widely touted as the best game ever made. Now its mechanics have been used by so many other games that it's just "good". Still very enjoyable, if overhyped.
The Witcher 2 is a live action RPG. It's a mature game with a solid story line. The story line branches out heavily. As in, the entire story is totally different (different sides, different locations, etc) based on a few choices that you make at points of the game. As a result, it takes multiple play throughs to see all the possible choices. Note that it's very computationally intensive. An open world sequel is coming out, soon, too.
And then there's various games that I haven't played that have strong reputations, such as Team Fortress 2 (multiplayer FPS), Left 4 Dead 2 (similar, but with zombies), CounterStrike (another multiplayer FPS), and Europa Universalis IV (strategy game that is supposed to have some of the largest time investments by players).
Linux's library pales compared to the Windows roster, but still some great games. And hopefully as Steam gets more popular on Linux, we can expect to see more developers porting their games.
