What are some "must have" games from GOG?

Descent Freespace and Freespace 2.

I loved Alpha Centauri - picked that up from GOG recently and having just as much fun playing it the second time around.

System Shock 2, Fallout, Duke Nukem 3D, Deus Ex, Roller Coaster Tycoon, Swat 3, Unreal Tournament Series. That's all that currently comes to mind.

Shogo: Mobile Armor Division. A fine game from Monolith, which later made FEAR games.

Unreal Tournament 2004

Fallout 1 and 2 + Alpha Centauri

cool

Strike Commander was added yesterday. Great story-based combat flight sim. Giants and Psychonauts are two of of my favourites from GOG. And, of course... System Shock 2.

Painkiller Black Edition, Fallout 1+2, Duke Nukem 3D, Arx Fatalis, Baldur's Gate 1+2, Deus Ex, Rayman 2, Thief Gold and Ultima Underworld. :)

Dungeon Keeper 2!

Nox, heroes of might magic 3, and dungeon keeper 2

PANZER GENERAL 2
and you already said HoMM3

Which games on GOG are "must haves?" Crimony... It's easier to say which games on GOG are NOT "must haves."

I would recommend you "Browse (all) Games" and then "sort by rating." Then, starting at the top and working your way down the list, pick out the game types you enjoy. If you have doubts, the GOG community is extremely helpful and will work with you to determine which games are right for you. It's almost a "can't lose" kind of proposition.

Also, many of the games I listed in another post are found on GOG.

Icewind Dale if you want some good dungeon crawling and the Fallout games of course.

Constructor, and Gangsters: Organized Crime.

Shogo M.A.D, bought it few months ago still a great game.

In addition to the Witcher series, Divine Divinity and Legend of Gimrock.

Original War. I love the damn game.

Gabriel Knight 1 and 2 are the two best games in GOG.

Edit: scratch that. They are the two best games in existence.

System Shock 2, Witcher and Witcher 2 and Fallout 2.
Fallout 2 is a must have RPG. War. War never changes.

Witcher and Witcher 2 the games you must have. I also recommend Baldur's Gate 1 & 2, Arcanum and Neverwinter Nights. ;)

It would probably be easier to name those that aren't must haves tbh :D

Dungeon Keeper series, total anhilation series, baldurs gate series, neverwinter night series, planescape torment, wing commander and fallout series