30 must-have downloadable games

From thecrankyhermit

Also, see 10 must-have non-downloadable games.

Age of Empires II: The Age of Kings

Where to get it: Steam

Another World

Where to get it: Good Old Games

Baldur's Gate: The Original Saga

Where to get it: Good Old Games
I can't really recommend playing the Enhanced Edition due to content exclusion, but it comes with the complete "Original Saga" when you buy it on GOG.

Baldur's Gate II: Shadows of Amn

Where to get it: Good Old Games
Likewise, the non-enhanced original is also included here.

Day of the Tentacle

Where to get it: Good Old Games

Deus Ex

Where to get it: Good Old Games
Recommended fan patch: Deus Exe. Get the DirectX 10 renderer there too.

Diablo II

Where to get it: Blizzard Store
Also consider Lord of Destruction.

DOOM

Where to get it: Good Old Games

I recommend using the GZDoom engine to play it.

To use it, download and install it to a directory, and call it "GZDoom."
Now download and install The Ultimate Doom, and browse to its install directory.
Find the file "DOOM.WAD." Copy it to the GZDoom directory.

You can now run GZDoom, and you can use it to play The Ultimate DOOM with loads of enhancements, like support for high resolutions, mouselook, and Internet multiplayer.

This also works with other DOOM engine games on GOG and Steam, namely DOOM II, Final Doom, Heretic, Hexen, and Strife. Find those games' main WAD files (sometimes there's more than one) and copy them to the GZDoom directory as well. If you have more than one game's WAD file(s) present, GZDoom will offer a menu for selecting a DOOM engine game to play.

The Elder Scrolls III: Morrowind

Where to get it: Good Old Games
The dated graphics can be quickly spruced up with the mod MGE XE, which enables widescreen resolutions, long draw distances, and DX9 enhanced lighting and shaders. Soul sucking registration required, and may tax anything less than a Core i5.

Fallout

Where to get it: Good Old Games
HOWEVER - the version of Fallout that GoG.com sells is censored! Specifically, there are no children in it. You can bring them back by applying the Fallout 1 Children Patch.

Fallout 2

Where to get it: Good Old Games
Unlike Fallout, GOG's version of Fallout 2 is not censored. But it is quite buggy. You can fix hundreds of these bugs by downloading killap's Unofficial Fallout 2 Patch.

Full Throttle

Where to get it: Good Old Games

Gabriel Knight: Sins of the Fathers

Where to get it: Good Old Games

Grim Fandango

Where to get it: Good Old Games

Half-Life 1 Anthology

Where to get it: Steam
Also consider Valve Complete Pack.
Skip Half-Life: Source. It is a feeble attempt at a remake and is buggier than the original.
You can, however, improve the graphics somewhat with the official High Definition Pack. It must be downloaded separately.
Also, the Steam version of Blue Shift has some bugs. These can be fixed with the Blue Shift Unlocked mod.
Finally, there is bonus co-op campaign called Half-Life: Decay, which officially is exclusive to the PS2 port. Decay has an unofficial PC port, which can be downloaded here.

Half-Life 2

Where to get it: Steam
Also consider The Orange Box and Valve Complete Pack.

Indiana Jones and the Fate of Atlantis

Where to get it: Good Old Games

Max Payne

Where to get it: Steam
Also consider Max Payne Bundle.

Monkey Island 2: LeChuck's Revenge

Where to get it: Good Old Games
One caveat - this is not the original version of the game, even in "classic" mode, which has some content cut and some compromises, the biggest being that the music in "classic" mode sounds totally wrong. But you can fix this by using ScummVM to play "classic" mode, and I highly recommend you play it this way your first time.
More details here.

Planescape: Torment

Where to get it: Good Old Games
For some reason, they've decided to increase the game's frame rate. I don't know why, but this causes the game's animations to run faster than they should, and I recommend changing it back to the default speed. To do this, go to the game's directory and edit the file "Torment.ini." Find this line:
Maximum Frame Rate=40
and delete it. Or change the number to 30.

Portal

Where to get it: Steam

Quake

Where to get it: Steam
Don't bother getting Quake by itself. The Steam version has some flaws, and you can fix them (I show you how in the links below), but you NEED the whole pack to fix the most serious flaw - the lack of music. This goes for Quake II as well.
You're playing it wrong: Quake
You're playing it wrong: Quake II

Sid Meier's Alpha Centauri

Where to get it: Good Old Games

Star Control II - The Ur-Quan Masters

Where to get it: sc2.sourceforge.net - this game is completely free to download and play.

Starcraft

Where to get it: Blizzard Store

System Shock 2

Where to get it: Good Old Games

Thief II: The Metal Age

Where to get it: Good Old Games

TIE Fighter

Where to get it: Good Old Games
There's three versions included; 1994 TIE Fighter, 1995 TIE Fighter "Collector's CD-ROM," and 1998 TIE Fighter "Collector Series." Be sure to play the 1995 DOS-based "Collector's CD-ROM," it's by far the best of the three.

Warcraft III: Reign of Chaos

Where to get it: Blizzard Store

X-COM: UFO Defense

Where to get it: Good Old Games