BENEATH A STEEL SKY, is free,but i love it.

Arcanum

Heroes of Might & Magic III Complete (although the first 4 games are available)
The Chronicles of Riddick: Assault on Dark Athena
Neverwinter Nights 1 & 2
Medal Of Honor Warchest
Clive Barker's Undying
But there are soo many good games on GOG that aren't on Steam and also cheap, that would be a shame not to buy :))

Guilty Gear X2: Reload

XIII
Even though that one is not on gog anymore either technically speaking due to some expiring rights or something. A crying shame since the game was largely underrated

Sid Meier's Alpha Centauri/Alien Crossfire and the entire Wing Commander series would be my choices.

Neverwinter nights series and Sanitarium. Neither GoG nor Steam have Grim Fandango but I have to b*ch about it xD

Clive Barker's Undying.

Outcast !
Hopefully Outcast Reboot HD becomes real and comes to Steam.

Neverwinter Nights Diamond, Blade of Darkness and Advent Rising are 3 of my favorites, but GOG has so many great vintage games it's hard to actually remember them. Those are just from the top of my head.

Neverwinter Nights Diamond Edition. It's really one of the best games I've played as of to-date.

Somehow nobody mentioned Jade Empire - it's a great game, and, unfortunately, it's unplayable on Steam.

Arcanum, Gabriel Knight series, Sanitarium, Stonekeep, Journeyman project series, Alpha Centauri, Zork games, Phantasmagoria series, Messiah, Vampire the masquerade: Redemption, Might & Magic series, Ultima series, Fahrenheit, Quest for glory series, Lands of lore series, Kyrandia series, Jade Empire and Neverwinter Nights 2 that used to be on steam, Bloodnet and others I guess :)

Shogo Mobile Armor Division!

Dungeon Keeper

Ultima 7 & Underworld
Arcanum
Sanitarium
Alpha Centauri
Might & Magic series (3-5, 7 & 8 in particular)
Wing Commander series
Dungeon Keeper series
Legend of Kyrandia
Beneath a Steel Sky
Panzer General 2
Battle Isle series
Gabriel Knight series
Little Big Adventure series

There are way too many games you should get from GOG to list here. Once upon a time, GOG's catalog was almost entirely four- and five-star games. Then they started adding some of the newer stuff, and not all of those games have been up to that standard. Even so, I would suggest browsing the entire GOG catalog--sorted by popularity--and wishlisting anything that looks interesting. Pretty much everything on GOG goes on sale from time to time, at sales prices up to 80% off.

In terms of which games are worth getting, the GOG community does a good job of reviewing and rating the games with only a couple of exceptions (e.g. Jack Keane 2). Anything with four or more stars is worth getting, and unlike Steam, they are tweaked to run on modern systems. Also, GOG offers Mac and Linux support. Finally, GOG games are truly DRM-free and can therefore be played anywhere, anytime, without needing any kind of platform or third-party software running.

PLANESCAPE: TORMENT

Theme Hospital,

Constructor,

Dungeon Keeper,

Dungeon Keeper 2

Did someone say GOG? Let's see...
Battle Isle
History Line: 1914-1918
Lure of the Temptress
Master of Orion
Battle Isle 2
Teenagent
Beneath a Steel Sky
Dragonsphere
SimCity 2000 Special Edition
Battle Isle 3
Flight of the Amazon Queen
Master of Orion 2
M.A.X.
Stargunner
Dungeon Keeper Gold
Incubation
Fallout
Carmageddon 2: Carpocalypse Now
M.A.X.2
Sanitarium
Fallout 2
King of Dragon Pass
Sid Meier's Alpha Centauri
Planescape: Torment
Baldur's Gate: The Original Saga
Tyrian 2000
Freespace 2
Hitman: Codename 47
Vampire The Masquerade: Redemption
Baldur's Gate 2 Complete
Capitalism 2
Disciples: Sacred Lands Gold
Icewind Dale Complete
Fallout Tactics
Icewind Dale 2 Complete
Disciples 2 Gold
BloodRayne
The Temple of Elemental Evil
Nexus: The Jupiter Incident
Forgotten Realms: Demon Stone
Dungeons & Dragons: Dragonshard
BloodRayne2
Fahrenheit (Indigo Prophecy)
Neverwinter Nights: Diamond Edition
Neverwinter Nights 2 Complete
Grid
Torchlight
Still Life 2
Treasure Adventure Game
Alan Wake's American Nightmare

A few of them are free.
And I wouldn't say GOG is rival to Steam. They have much smaller catalog, most of the games are oldies, and I don't remember Valve actually working together with the developers to adapt old games for modern systems, fix bugs and even add minor features based on users feedback - and I saw that happening on GOG. While old games on Steam often require little tampering for comfortable play, I never had that with GOG, it's like their trademark to me.
Last, not least, all GOG games are entirely DRM-free.

http://www.steamgifts.com/forum/e4O9P/what-are-the-best-gog-titles-that-are-not-on-steam :D

A lot really. To name a few: Krondor, Sanitarium, the first games from the Heroes series, Quest games, old city builders like Caesar, Cleopatra etc., seconding Shogo and plenty more I'm forgetting right now. But yeah, GOG a great. Galaxy can't come out soon enough.

Take a deep breath ... Lego Rock Raiders! What? In my opinion it was a dang good game for wasting time and the controls weren't bad either. And it is much better compaired to the games that were relased by Lego in that time era.

Planescape Torment
Baldur's Gate Series (Old)
Neverwinter Nights
Jade Empire
Arcanum: Of Steamworks and Magick Obscura
Heroes of Might & Magic III
Master of Orion II