Grim Fandango. Period.
No other game in the history of mankind has been so goddamn good.....and yet so thoroughly underrated and ignored when it came out.

It had two stumbling blocks, one: It was part of the era of 3D transitioning adventure games, of which many were poor, so people were wary. Two: It was also at a time when adventure games in general were not appreciated.
I have yet to play it, missed it at the time, cant even remember why now.

Just getting the word out. :P

Most of them are not underrated nor unknown, but Jet Set Radio is f*cking awesome.

1. Call of Cthulhu: Dark Corners of the Earth
2. Psychonauts
3. Grim Fandango
4. Titan Quest
5. The Witcher

King's Bounty : The Legend.
Psychonauts
Silent Storm
Tropico
Fantasy General

Blasphemy! You lacked the will, thats the problem! Your pennance is to look on ebay for a copy, buy it, play it (using the installer & launcher on this page) and say "Shit, this game is AWESOME!"

no particular order:
Sword of the Stars (nicely put together and easy to learn space civ strategy thing... kind of like a halfway point between GalCiv2 and SoaSE)
Beyond Good and Evil (likeable supporting characters? No surely not! Some wonderful gameplay and exploration with Uncle Pey'j, the best pig ever)
Enemy Territory: Quake Wars (why on earth there aren't more people playing this I don't understand. One of the funnest team shooters I have played. Also jetpacks!)
Rise of Legends (Its a fantasy game (with a bit of sci-fi) but completely lack elves and orcs. Which is a little bizzarre but quite refreshing. few games have done steampunk better)
Nexus the Jupiter Incident (Very solid and enjoyable Space RTS)
Warzone 2100 (played it on playstation and it wasnt particualrly popular on there dont know what it was like on PC but it is still one of the better RTS's I have played with a lot of very good and original ideas that I havent seen done better (or at all) since)
Well thats all I can think of just now

*Alien Hominid (2D shooter that came out on the Gamecube, Xbox, and PS2, depending on where you live)
*Ikaruga (top-down shooter that looks stunning and is extremely hard)
*Golden Sun 1 & 2 (RPG on the GBA. One of the most enjoyable RPGs I've ever played)
*Knights of the Old Republic (fairly well known among gamers, but it's not Halo in terms of popularity. One of my favorites of all time)
I've really been wanting to play Grim Fandango. I'm hoping that GOG gets it someday (come on LucasArts!). I'm running Vista 64 bit, so even if I do find it, I'd have to use some workaround to play it.

Death to Spies - a cool WW2 game that took the game from a Soviet perspective amongst the glut of American-themed WW2 games. You could be a stealthy character who could use disguises to get to his objective You could hide bodies in boxes, in trucks and anywhere else. You could even look through keyholes before you entered a room to ascertain the situation inside. It did have some flaws but what game doesn't?
Master of Magic - a turn based strategy game that never got the love it deserved. I liked how you could use the different varieties of captured races as units. The end objective was pretty unique: instead of obliterating your enemies, your main objective was to cast the biggest, longest casting spell and gain dominion over all.
The Legacy: Realm of Terror - horror themed single player single character RPG done in a sort of EOB style except in sort of real-time. It is still quite unique today since almost no one makes horror themed RPGs anymore.
Darklands - open-ended RPG set in a semi-mythical medieval Germany. You could do anything you wanted in that game and always find a quest or a challenge to conquer. One of the few games to use religion in a non-D&D way with praying for a saint's favor.

Taipan (played on an Apple IIe back in 1988; sail your ship around the Orient trading goods in a changing economy; it was mostly text based, though attacking pirates were represented by static silhouettes of ships that disappeared as you sank them)
Betrayal at Krondor (PC-DOS, 1993, made by Dynamix (Sierra); RPG with story by fantasy author Raymond E. Feist, using characters from his early novels)
Imperialism (Windows, 1997, by SSI; TBS; might be considered slightly popular, but I think it didn't received the popularity it deserved)
Heroes of Might and Magic II seems obscure today, but probably wasn't in its day.
Ladybug (played on the CoCo2 about 1984; PacMan clone, but sections of the walls would rotate, so you could alter the maze and block your pursuers)
My copy of Ladybug on cassette tape had a volume drop in the middle of the program, so while loading it into the computer, I had to toggle "audio on" and listen to the screeching noise, then quickly move the volume knob up when it happened. Ah the "good old days." :D

http://quick.mixnmojo.com/software thats a workaround for install & playing in x64 systems.
Grim Fandango would definitely top my list of games people should play

1. Chaser
2. Heroes of Anihilated Empires
3. Experience 112

Ugh... another list thread? The problem with these types of posts is the lack of focus. Some people just make uninteresting list that offer no real substantial content. Then others try to discuss each item on their list making for really long posts that get lost in clutter of all the other posts.
And people complain about the quality of my threads...