Breakdown (Xbox)
Deadlock: Planetary Conquest (PC)
Gungriffon (Saturn)
Original War (PC)
The Red Star (PS2)
Space Giraffe (X360)
WarTech: Senko no Ronde (X360)
A lot of the games listed in this thread aren't exactly underrated. For example Psyconauts might have sold poorly but critically it was a success.

Underrated by the people.

I effing love that game.
Some of mine:
Journey to Silius
Power Stone
Psychonauts
ES II: Daggerfall (which is up on the wishlist!)
The Neverhood
Skullmonkeys

dreamfall.
Underrated by everyone. Amazing 'game' which should be treated more as interactive book than a standard adventure game.

Fahrenheit was really good, I don't know if it sold that well.

I hear it sold OK, didn't break records, but held its own.

I found Fahrenheit (Indigo Prophecy) to be a bad game, with horrible super natural story and an ever worse gameplay based on QTE.

While I hate QTEs, I actually didn't hate fahrenheit for some reason. The story DID go quite dodgy towards the end

1. Blade Runner, one of the best adventure games ever!
2. Grim Fandango, another one of the best
3. Vampire the masqurade: Bloodlines, great rpg a bit buggy but can be fixed with patch :)
4. Fahrenheit
Im quite sure I have some more around here, but I cant remember anyone right now

Why have jetpacks when you can have... jetboots!?
How about Armed and Dangerous. By the same people who made Giants, amazing gameplay, plenty of action, solid twisted humour the whole way through, brilliant soundtrack.

1) Thief
2) Broken Sword
3) Stronghold
4) Final Liberation
5) Might and Magic

Paraworld
Zanzarah: The Hidden Portal
The Wheel of Time
Metal Fatigue
Just Cause
Joint Task Force
Drakan
Shadow Man
Need for Speed: Porsche 2000
Dreamfall

1) Enclave - PC
2) Hotel Dusk: Room 215 - DS
3) Chornicals of Riddick: Escape from Butcher Bay - PC
4) The Longest Jouney - PC
5) Sanitarium - PC
6) Elite Beat Agents - DS
7) Keepsake - PC
8) Still Life - PC
9) Safe Cracker - PC
10) Riven - PC
I'd totally agree with zuhutay on 'Dreamfall' too, great adventure game which people got bored of too quickly

Project Eden is another gem that has been passed over. You control a squad of 4 people in a dystopian cyberpunk future (The Future! Now with RealMeat!). The game has some unique puzzles where you have to use 2 or more of your squad's members in order to get past obstacles and use devices. The game also features combat, great dialogue, and some awesome atmospere. Sadly, it's early 3d graphics might put some people off but the game isn't about the leet grapfix but about the story and gameplay.