Asheron's Call. By far the best MMO to date in my opinion (and the opinion of nearly everyone that has played it for a good chunk of time). It was fairly popular in it's heyday (2000-2004ish), but it wasn't as well known as Ultima Online, Lineage, Everquest, and of course, WoW.
Crysis. Reviewers may have liked it, but a lot of people here seem to criticize it. It's not the best game, but pretty fun. I've wasted $50 on worse things before.
Doom 3. Same as above - critics were fairly friendly towards it, but people here bash it all the time. I liked it. What FPS at the time was vastly superior? Sure HL2 brought a far better story and more interesting environment to the table, but Doom 3 really pushed graphics to a new level and did a good job of scaring the crap out of you from time to time.
Amnesia: The Dark Descent. I STILL have not finished this game because it scares the living daylights out of me. I mean, it's not like I'm jumping out of my seat every second and screaming like a twelve year old Catholic school girl (no offense to twelve year old Catholic school girls, of course), but it makes me so tense waiting for the next thing to fall out of the ceiling or let out a blood-curdling cry and run at me trying to rip my head off. After ~30 minutes of playing, I have to turn it off and do something different.
Trine. All around wonderful game. I paid $5 for it, and it was EASILY worth $20.
Titan Quest / Immortal Throne. Buggy, blatant Diablo II clone in a different setting, etc, but overall a great hack-and-slash with a good loot system. My only complaint is that the game needs more skills / spells, like Diablo II.