QUAKE 1 + EXPANSION PACKS + NEHAHRA

A lot of indie games that are cheap on Steam are well-worth the short time investment:

Dear Esther (if you're into that sort of thing, its only about an hour long and it isn't really a game by definition)
Bastion (great atmosphere, really cool story-telling mechanic)
Limbo (awesome atmosphere and good puzzles)

As for others:

Portal 1/2 (you can run through both of them in about 10 hours tops)
Mirror's Edge (also short)
Fallout 3 or NV if you have that kind of time on your hands
Deus Ex: Human Revolutions
Civilization V
Bioshock
Dead Space
Mass Effect

Dead space 1 and 2 definatly worth playing through, amazing games. Can't wait for the 3rd!

system shock 2, entire hl series, penumbra 1 and 2, splinter cell chaos theory (best of the series by far) and for gods sake limbo

i like ur taste

carmageddon + splat pack

STALKER series using the Complete mods. You could probably skip Clear Sky as contains mostly the same areas as shadow of Chernobyl.

Check out this neat little list also:

http://www.esreality.com/post/2223413/#pid2223413

The Binding of Isaac - GOTY 2011 get it!!!

the only ones worth playing are portal series and the mass effect series.

Try point & click adventures theres plenty of good games there

plenty is an understatement!

I agree. TC, play the Myst series.

point and click are the best kind of solo game

Try point & click adventures theres plenty of good games there

point and click are the best kind of solo game

plenty is an understatement!

understatement is an understatement!

Understating that understatement is in itself an understatement!

Fallout
Fallout 2
Fallout 3
Planescape Torment
Baldur's Gate
Baldur's Gate 2
+all expansions
Star Wars: Knights of the Old Republic
Star Wars: Knights of the Old Republic II
Final Fantasy VII
Final Fantasy VIII

Starcraft + Brood War
Warcraft III RoC + TFT

Heroes of Might & Magic III
Age of Wonders
Age of Wonders II

Carmageddon
Carmageddon 2

Need for Speed
Need for Speed: High Stakes
Need for Speed: Porsche Unleashed

Duke Nukem 3D
Doom
Doom II: Hell on Earth
Doom 3
Quake + all expansions
Quake II + all expansions
Quake 4
Aliens vs. Predator (the original)
Half-Life + Blueshift
Half-Life 2
S.T.A.L.K.E.R.
Far Cry 2

I guess this will last for quite a long time. :)

apparently you only play shit games so I won't bother suggesting New Vegas.