Arcanum. I remember a long time ago I read a review for this game somewhere when I was looking into the Fallout games, but other than that, I knew nothing about the game. It went on a promo here on GOG, though, and I picked it up. I don't regret it at all.
Septerra Core. I really had never heard of this one before, and while I have yet to finish it, I consider it to be one of the better games I've gotten here.
realMyst. Okay, I don't consider this game to be the greatest thing ever, nor do I truly believe it is the greatest adventure ever to be created. But, I did enjoy it a great deal, and I wouldn't have been able to do that without GOG.
Beneath a Steel Sky and Broken Sword. I am usually very leery of adventure games, and I'll admit I would never have purchased these. But having gotten them both for free (BaSS because it is permanently free, Broken Sword because it was free for a while), I am glad to say had I paid for them, I wouldn't regret the purchase.
Giants: Citizen Kabuto. Do I need to explain this one?