system shock 2,
eador - genesis (must be if you like HoMM),
age of wonder 1&2,
capitalism 2 (hell yeah 4 making money)

Heroes of Might & Magic III. Populous: The Beginning?

Nobody mentioned Prince Of Persia sands of time trilogy? for fucking shame.

System Shock 2 and Heroes 3: Might and Magic.

System shock 2, Omikron, Gabriel Knight series, Lands of lore, Master of orion 1+2, Sanitarium, Zork games, Alpha centauri, Outcast, Septerra core, Arcanum, Feeble files, Quest for glory, Theme hospital, Dungeon keeper and so on :)

Psychonauts.

There are no "must have" games anywhere. No one "needs" games in order to live.

• Rollercoaster Tycoon 1 and 2
• Gobliiins
• Ground Control (Note - you can get the full version for free elsewhere, but it doesn't include the expansion. GOG version does)
• Beyond Good and Evil

Besides them having dropped the 'Good Old Games' tag way back. Give the Rayman series a whirl if you enjoy platformers.

Might & Magic Series

Planscape: Torment
Bladurs Gate series

Icewind Dale series

• System Shock 2
• Divinity series
• Age of Wonders series
• Longest Journey series
• Another World

And of course, Torment and Baldur's Gate as you have mentioned.

King of Dragon Pass. Not a lot of games like it and I've enjoyed it immensely.

Sanitarium, Planescape Torment, Icewind Dale and Baldurs Gate series. <--- these are like the best games ever!

Lords of the Realm, a great old strategy game and any of the Rayman games, especially Rayman 2, that is a personal favorite of mine.

Far too many, so prepare your wallet.

Baldur's Gate 1 & 2, Planescape: Torment, Fallout 1-2, The Witcher 1-2, Arcanum, Master of Orion 2, Alpha Centauri, Theme Hospital, Dungeon Keeper 1, Icewind Dale 1-2, Temple of Elemental Evil (though less important than the infinity engine games), Ultima 7, Legend of Grimrock, Might & Magic 4-8, System Shock 2, Mater of Magic, Myst 1 (either version), Riven, Outcast, Gabriel Knight 1-3, Divine Divinity, Wing Commandder 2-4, Ultima Underworld 1-2, Sanitarium, Freepsace 1-2, Sacrifice, The Longest Journey, Jagged Alliance 2, Lords of the Realm 2, Crusader no Remorse & no Regret, King of Dragon pass, Thief 1-3, Deus Ex, Age of Wonders 1 & Shadow Magic, Syberia 1-2, Betrayal at Krondor, Magic Carpet, Settlers 2 (either version), Arx Fatalis, Lans of Lore 1, Gothic 1-3, Little Big Adventure 1-2, Panzer General 2, The Last Express, Geneforge 1-5, Independence War 1-2, Tex Murphy 3-5, IL-2 Sturmovik, Battle Isle, Realms of Arkania 1-3, realms of teh Haunting, SWAT 3, Avernum: The Complete Saga & Combat Mission.

Tried to get a good mix of genres in there. I also tried to pick games that has some historical importance but that are also fun to play these days (so no Ultima 1-3)

Planescape Torment: Baldur's Gate 1/2, Fallout 1/2, Arcanum, Clive Barker's Undying, Blood, Shadow Warrior

Planescape was kinda good

As now I think only of Planescape: Torment, which is a masterpiece, but it can be on Steam in the future so dunno.

There's just way too many good ones. Wait for sales and look up the ones that seem interesting. Right now Blitzkrieg series is on sale and apparently the first is the best. Here's the catalog, sort by User Rating or Best Selling.

My personal favorites are:
Wing Commander series (bought them for the nostalgia. Hope they still hold up)
Alpha Centauri
Freespace 2
Combat Mission: Beyond Overlord (tried it recently and was pleasantly surprised. The control method is so outdated though.)
The Settlers series (I bought Settlers 2 but haven't touched it yet. I played 6 or 7 and thought it was great, and they say 2 is best)
Arcanum (the only RPG I've tried so far from GOG. It was magical! I'm old and this made me feel like a little kid again exploring his first RPG. And they say Planetscape and Baldur's Gate are even better games. Can't wait to try those too.)

haven't played it but heard good things about masters of magic

My personal GoG favorites:

• Baldur's Gate 2
• Arcanum
• Icewind Dale 2
• Planescape Torment
• Jagger Alliance
• Dragonsphere (I wonder if it stills free)

this one D35E-U5AB-4W34-58HK

"Crusader: No regret"! One of my favorite games ever! Also another must have is the "Alone in the Dark Trilogy" and the outstanding "Robinson's Requiem".

Roller Coaster Tycoon Deluxe!