I'm a bit late to the happy birthday wishes, but I will throw a few games to the table. We all know the old classics, as most are on GOG, and then there's a bunch of good and interesting indie adventure games, and a few names were dropped already. I will focus mostly, though, on adventure games I'm interested on, but are not on GOG because they mostly ignore adventures past lucasarts and pre-kickstarter.
As DeMignon mentioned Ripley's believe it or not, I'll throw
<span class="bold">Orion Burger (1996)</span>, from the same developers.
Then there are the Psygnosis games, the last one being
<span class="bold">The Gene Machine (1996)</span>. Those are Sony's know, methinks, so we low changes on getting here ever.
There's always Coktel Vision's wacky
<span class="bold">The Bizarre Adventures Of Woodruff and The Schnibble of Azimuth (1995)</span>.
Obviously, many people here wants
<span class="bold">Star Trek The Next Generation: A Final Unity (1995)</span>.
There's a very unknown and ignored classic in
<span class="bold">Duckman: The Graphic Adventures of a Private Dick (1997)</span>.
And I think
<span class="bold">Nightlong: Union City Conspiracy (1998)</span> is a pretty worthy adventure.
The developers of the first Black Mirror, Future Games, also did a bunch of other adventures. The first after BM was
<span class="bold">Nibiru (2005)</span>.
Wizarbox, developers of Greymatter (minus Jane Jensen) released
<span class="bold">So Blonde (2008)</span>.
Autumn Moon, from Curse Of Monkey Island's Bill Tyler, released
<span class="bold">A Vampyre Story (2008)</span>.
I heard good things about Centauri Production's
<span class="bold">Memento Mori 2 (2012)</span>, though adventure gamer's review didn't praise it as much (it got good mentions in the forums).
From Animation Arts, from the Secret Files series I will highlight
<span class="bold">Lost Horizon (2010)</span>.
Cranberry Productions, the guys behind Black Mirror II and III also did
<span class="bold">Lost Chronicles Of Zerzura (2012)</span>.
Pendulo studios, apart from the Runaway series here on GOG, also have several other adventures, the last of which was
<span class="bold">Yesterday (2012)</span>.
Silver Style are not the most liked developer, they who did Simon The Sorcerer 4 & 5, but I always thought
<span class="bold">Goin' Downtown (2008)</span> had an interesting setup.
The nice guys of Himalaya Studios released their first comertial adventure
<span class="bold">Al Emmo And The Lost Dutchman's Mine (2006)</span> after their free Sierra remakes.
Another classic style from the years post adventure games is
<span class="bold">Gilbert Goodmate and the Mushroom Of Phungoria (2001)</span>. I think it got released in Steam, but not in GOG..
And one that I always mention and few people know, and I remember it got good reviews back in the day is the scifi
<span class="bold">The Immortals Of Terra: A Perry Rhodan Adventure (2008)</span>.
The sad thing is that some of those games could probably be released in GOG, as the publishers and in here already. GOG just doesn't seem all that interested in games that probably won't sell as much as the sierra and lucasarts classics. So for all those whining that GOG is releasing adventure games all the time... NO, they are NOT. And this is only a small sample of it.