The two that immediately spring to mind are Psychonauts and Hostile Waters: Antaeus Rising (sadly no longer available here). If I scroll through my games list, I may find a few more.

Never played that game. What's it like?

Just as I thought. Looking through my list brought forth a few more.

Advent Rising
Darkstar One
Postal 2
Serious Sam: The First Encounter

Cartoonish cell-shaded shooter. Excellent gameplay and atmosphere. Very varied and well-designed levels, sometimes allowing for sneak vs gung-ho approaches. It's a bit nolf-like in feel (not as much witty-dialogue-driven). Quite unforgettable. You have the ability to take people hostage, when you snipe someone you have a few comic-book vignettes showing the effects. It's brutal, yet graphically feels like being immersed in a cartoon or comic book, yet feel real and flows very naturally. It's just fantastically stylish, and fantastically enjoyable, gameplay-wise.

Only flaw : an unresolved cliffhanger at the end.

Might & Magic 1&2 except the manual mapping...
Outcast

Odd. I never bothered with it, because I bought a PS2 copy of the game on a whim, and absolutely hated it. It was an uncontrollable mess, but then so are most console FPS games. I always assumed that the PC version would be a shitty console port with shitty controls. It seems you are saying that this is not the case. If so, I may have to check it out.

Great old games I discovered and played on gog:
Little Big Adventure
Tex Murphy (thanks again Bigs for the chance to play these o/)
Master of Magic
Star Control 2

Great old games I discovered on gog, but didn't play yet:
Betrayal at Krondor
Master of Orion

A special case is Heroes of Might and Magic 3. I had the game (without the expansions) for years, but didn't like it (don't ask me why). When gog released HoMM5 I installed it again to see why I hated it, and fell in love with the game. Two days later gog made a Might and Magic Weekend promo and I got the complete version. So, I didn't "discover it on gog", but "thanks to gog" :)

Well yes, I consider it one of the 2 or 3 best fps ever, on the PC. Along with NOLF, and the first AvP. And no nostalgia, as I've played it quite recently.

I'm 17, which means I was too young/not born when most of the stuff on here came out. Which means that since I discovered the site I've suddenly been exposed to some of the best games of all time all at once and it's awesome. :)

More specifically though, here're some that've ended up on my top games of all time list: Arcanum (Probably top 3 games), The Longest Journey (Also probably top 3), Sid Meier's Alpha Centauri, The Last Express, Fallout, Tex Murphy: The Pandora Directive. Now I've got to go finish Planescape and Fallout 2 so I can put them on this list. :)
Welp, that's another item on the ol' wishlist. :)

At the risk of sounding like an old fart (I just turned 35), I'll say that it warms my heart that some teenagers today can play these "golden oldies" and appreciate them for what they are, without for instance having to compare the graphics to modern games.

Old games that I enjoyed, which I played for the first time on GOG:

Fallout
Psychonauts
Septerra Core
Blake Stone
XIII

This 21 year-old would like to remind you that you forgot Sacrifice :)

Well, I didn't mean to say I haven't played many 90's games, it's just that I've played most of the ones that interested me way before discovering GOG. ;)

But my last PC lasted from about 1998 to 2007 and it was a low end one, meaning that I couldn't play a lot of games from 2000 on or so. And several of those didn't even start on my system despite my PC meeting the official system requirements; not sure if due to the copy protection on the disc (I used original discs, no copies!) or Windows compatibility issues. E.g. I never got the original Gothic games to run on my Win98 PC, so I'm glad GOG allowed me to catch up on those (although Gothic 2 isn't really fun to play with my 2007 onboard graphic card either, d'oh!).

Woah, i didn't realise you were that age. I thought you were younger.