I second wolf's recommendations of the GK games(especially 1 and 3) and the Blackwell games...both series have nicely done scenes, music, characters.

I also second Ketobak's recommendation of The Journey Down games...very nice games with a unique Jamaican theme/feel...and Grim Fandango is a pretty nice one as well(the new version can be a bit slow on older PCs, though).

Pixelboy's recommendation of the Interplay Star Trek games I also concur with....though a word of caution: the one game sometimes becomes unbeatable in the last mission, so saving often is a good idea(such as right before that mission starts).

Aplomando's recommendation of the Monkey Island games is another I agree with and put my backing to: I love the theme, the music, the characters....all(beyond things like the one maze in one of the earlier games and the weird controls for the 4th game) good.

Breja's Picks of Indiana Jones Fate of Atlantis and the Deponia series are also very good as well...Indiana being more action-y and "old school", and the Deponia games being a more quirky zany comedic style of game.

Glaucos's pick of The Dig is also good....albeit the puzzles can be hard to figure out on a number of occasions and there is a bit of backtracking which can make finishing it a real task. Still, the intro/music/setting are very nice imo.

Lastly, I concur with the recommendation for Beneath a Steel Sky....very nice game(albeit with a bit of walking back and forth sometimes), with nice backgrounds/voice acting/characters/world.

(might've missed some)

One good source I found is gameboomers.com. They have the annual games list that has the classics and some of the newer games. I agree with lots of the games suggested, the only one I would add is The Longest Journey which is a nice early game.

I 2nd Quest for Glory. There is a free fan game for the 2nd one (to be played after the first) that improves upon the original with better graphics/UI:

http://www.agdinteractive.com/games/qfg2/homepage/homepage.html

They're aimed at children but check out Pajama Sam Vol. 1 (bundle with 2 adventure games and one action game) here. It's a better deal than PS Vol. 2, though PS V2 does have "You Are What You Eat" and that's a great game.

https://www.gog.com/game/pajama_sam_vol_1 ,
https://www.gog.com/game/pajama_sam_vol_2

(I should also point out that unfortunately they're English-only, unlike on Steam)

Not mentioned yet: if you like the sci-fi and cyberpunk genres with detective spin, I can recommend Gemini Rue and Technobabylon from Wadjet Eye.

I've already mentioned games from Wadjet Eye Games Studio.
P.S. Their Gemini Rue - fantastic in all means!

If OP also loved Dark Seed, maybe he should take a look at these:

https://www.gog.com/game/sanitarium
https://www.gog.com/game/i_have_no_mouth_and_i_must_scream
https://www.gog.com/game/harvester
https://www.gog.com/game/the_cat_lady
https://www.gog.com/game/phantasmagoria
https://www.gog.com/game/call_of_cthulhu_shadow_of_the_comet
https://www.gog.com/game/last_door_collectors_edition_the
https://www.gog.com/game/the_black_mirror

Lost Horizon is a rather nice Indiana Jones style game. It also has a sequel although that one was less well received.

yes i have the collectors set :) so unless they made one after 5 i have all of them.

I couldn't get into Gemini Rue. But Technobabylon is fantastic on all levels.

No one mentioned Blade Runner?

Philistines!

Amanita has it covered!