Fully Ramblomatic is the game studio of designer Ben "Yahtzee" Croshaw, and home to several of my favorites: The Trials of Odysseus Kent, a gothic black-humor adventure with stylish minimalist graphics (winner of a 2003 AGS Award for Best Dialogue Scripting); the outstanding, multiple award-winning John DeFoe quadrilogy, an era-spanning ghost story/murder mystery starting with whodunnit 5 Days a Stranger and continuing with the starship thriller 7 Days a Skeptic (concluding with new games I haven't yet had the pleasure of playing, Trilby's Notes and 6 Days a Sacrifice); and, one of his earlier endeavors, the Rob Blanc Trilogy, a comic space romp in the spirit of Douglas Adams. There are numerous other games there, including several new ones I have yet to play, and with Yahtzee's signature blend of compelling plot, wry humor, clever dialogue, well-executed puzzles, and respect for the adventure genre, they're pretty much all sure bets.
From Herculean Effort Productions comes the sterling Apprentice series, following the adventures of young magician's apprentice Mortimer Pibsworth. With two titles down and a third in the works, this series has received numerous AGS Awards and features lovely animation, fully voiced characters, and great gameplay and storytelling, including plenty of humor. Not to be missed.
xii games is a game studio with a couple of nice, offbeat titles to offer: by Vince Twelve, there is Anna, a talky, philosophical game in which you must restart the computer systems of your AI companion, and puzzler What Linus Bruckman Sees When His Eyes Are Closed, an interesting experiment where the screen is "doubled" into two visions of reality; there is also award-winning Spooks by Erin Robinson aka The Ivy, a classic, clever adventure set in the Land of the Dead.
Mourir en Mer by Dorcan is a touching, melancholy little game where you must help a dying invalid escape imprisonment by his tyrannical father and make a journey to the sea so that he can fulfill his dream to see it before he dies. Originally written in French, the English translation is not flawless, but the nice animation and challenging gameplay make this a solid, enjoyable game.
Permanent Daylight is a fun, irreverent adventure by Linus Larsson aka 2ma2 where you must stop an evil scientist from executing his plan to halt the sun in the sky, creating a world of permanent daylight. Winner of Best Character Art 2001, this is an all-around good game.
In addition to the few named above, the AGS site has tons and tons of games to download, along with user ratings and reviews. You can search or browse by title, length, or, if you want some good recommendations, start with the Picks of the Month or Award Winners page. There are quite a few titles I've had my eye on but haven't had the chance to play yet, and new ones are being added faster than I can keep up. There's certainly plenty to see.