I only have two (and the other one isn't even that old) + two debatable out of my collection of 37 GOGs. Hey, I'm mostly a nostalgia guy, therefor most of my games are ones I've played in the past. Although I have a couple of new ones too in there. But anyway,
-Broken Sword 1. Got this for free when GOG, for a limited time, gave it away for free. Now, I have played BS 3 or 4 (can't remember which) in the past, but never BS 1. But when I finally played it, I really, REALLY liked it and would even go so far as to say it's my favorite point 'n click adventure game of all-time. Without GOG I don't think I would've ever noticed this gem.
-Alien Shooter 2; perhaps not so much because of GOG, but a friend recommended it a year ago so I bought it. While it was nothing unbelievably spectacular, I still liked it a lot. But, if there was no GOG.com, I probably would have never played this one either.
Now the debatable ones are the Icewind Dale games. I had played IWD2 back when it was fresh, but for some reason (maybe the early difficulty plus me not studying the AD&D rules beforehand) I quit the game early and barely made past Targos. The enthusiasm of certain individuals of the GOG community however made me want to give the game another shot not long ago, but this time I also had the manual to go with it. So, I read it thoroughly and only after then started the game. And it was really, really good, I enjoyed it from the beginning to end. I got IWD1 yesterday, and although I'm only two hours in, that game looks really good as well (IWD1 I hadn't played in the past at all, mind you). These D&D games are looking to be so wonderful, I'm considering buying and trying out Baldur's Gate too after I'm done with IWD1. Yeah, it's true, I've never played BG. A huge gap in my general gaming knowledge, apparently. But anyway, I guess it's thanks to GOG why I've recently gotten into these best-known D&D games.