If you have no played them yet then here are some adventure games I HIGHLY suggest, which you can find on gog.
Gabriel Knight series - This is an adventure classic. Great mystery games (both serious at times and also with some comedy here and there), Gabriel is a great memorable lead character and the series itself is unique because it spans so many different presentations of the medium, the first one being a classic 2d point and click, the second one featuring a live FMV style adventure game, and the third one being a 3d based adventure game.
Sanitarium - This is without a doubt one of the CREEPIEST games I have ever played, even compared to the "survival horror" genre of games. It has a brilliant creepy atmosphere that will get under your skin. If you like horror themed games you MUST play this.
Still Life - EXCELLENT modern mystery adventure game. A pseudo sequel'ish to the Post Mortem adventure game (which you can also find on here). This is a serious themed murder/mystery game. There's a sequel to it , Still life II, but it's not as good as this one.
Syberia I and II - One of my favorite adventure games, great story, excellent art direction, very memorable game. They are also making a new one in the series. This game had very strong emotions throughout it, one of those games you'll remember a long long time after you've played it. Can't wait for the next one.
Also outside of gog, I'd recommend Gray Matter. It's a newer adventure game from Jane Jensen (Gabriel Knight creator) and is quite good, it is also similar in tone/style, a good mystery game but also a good love story.