Have Riddick, Planescape, Ultima, and a few more in my wishlist. Was wondering what other great games I should think about getting on GOG in the future. Thanks for replies.

Well, there are the recently released Forgotten Realms Archive Collections.

Might and Magic I - VI
Theme Hospital
Theme Park

Teenagent

(I'm serious).

None of the Might & Magic mainline titles up to 9 are on Steam, although they are available on UPlay.

None of the EA/Bullfrog classics are on Steam, but they are on Origin.

The old Silmarils games (Ishar, Robinson's Requiem) are not on Steam, but are available from DotEmu.

The SSI Gold Box games, Fantasy General and Pacific General (and, as Judas pointed out, the Warhammer games) are permanent GOG exclusives.

Many of the classics like the Digital Illusions/Spidersoft Pinball games, the Sierra adventures, some older Activision titles like the Zork games, many Interplay titles like Castles and Starflight, a few LucasArts titles (Rebel Assault 1 & 2), and some of the freebies (Teenagent, Beneath a Steel Sky, Lure of the Temptress) are GOG exclusives, but may not stay that way. Activision in particular tends to not be very keen on releasing classic titles on Steam.

Conventional wisdom suggests that classic games do not sell well on Steam, which is why they end up bundled on Bundle Stars and Indie Gala pretty sharpish. I suppose that's why Valve has stemmed the flow of classic releases of late.

I can't believe there's no GOGmix for this.

Thanks for the replies! Added everything mentioned to my wishlist and will be getting some of those in the near future.

None of the Hasbro games are available on Steam (though Steam does have the Enhanced Editions of Baldur's Gate 1+2 and Icewind Dale).

Also, while Steam does have a version of Heroes of Might and Magic 3, it's lacking the two expansions that GOG's version has.

Anno 1602
Anno 1503
Anno 1701
Arcanum: of Steamworks and Magic
Battle Realms (Soon to be added to Steam)
Conquest Frontier Wars
Clive Barker's Undying
Empire Earth
Empire Earth 2
Empire Earth 3
Evolva
Krush Kill 'n' Destroy Xtreme
KKnD 2 Krossfire
Medal of Honor Allied Assault War Chest
Nox
Rayman
Rayman 2
Rayman 3
Sea Dogs
Star Wars Galactic Battlegrounds
Total Annihilation
Total Annihilation Kingdoms
Total Overdose: A Gunslinger's Tale in Mexico
Warlords Battlecry
Warlords Battlecry 2
Warlords Battlecry 3

A few titles that come to mind:

Arcanum: Of Steamworks and Magick Obscura
Albion
Anvil of Dawn
Battle Realms
Betrayal at Krondor Pack
Clive Barker's Undying
Die by the Sword
Dungeon Keeper 1&2
Entomorph: Plague of the Dawnfall
Giants: Citizen Kabuto
Gobliiins 1-3
Little Big Adventure 1&2
Lands of Lore 1-3
Legend of Kyrandia 1-3
Lionheart: Legacy of the Creator
Neverwinter Nights 1&2
Normality
Nox
Outlaws
Rayman 1-3
Real Texas, The
Revenant
Silver
Stonekeep
The Temple of Elemental Evil
Vampire The Masquerade: Redemption
Wing Commander 1-4 & Privateer
all Zork games

If I'm not mistaken, Shogo: Mobile Armor Division isn't on Steam, either.

Beneath a Steel Sky and Lure of the Temptress are not GOG exclusives. Both are freely available for download from the ScummVM homepage, and have been for longer than they have been on GOG (as far as I know).

Wow, at first I chuckled when reading the title, because I thought there isn't anything that is not available on Steam but I'm completely baffled now. Why would anyone want to use Steam when it doesn't even has those gems?

Obviously, because you could also list a lot of great games on Steam not available on GOG and the choice from which store you buy doesn't have to be an exclusive either/or choice - you can get the best of both worlds. The DRM is a much more understable reason not to buy from Steam than Steam not offering some of these good old games.