Very nice: Samorost
http://analogik.org/samorost.asp

Just one Room, but...
http://www.datacraft.co.jp/takagism/index_e.html

A bit strange...
http://www.boohbah.com/zone.html

Oh, and did you see this 96k-gem? (needs a GeForce4:
http://kk.kema.at/files/kkrieger-beta.zip

Infos:
http://www.theprodukkt.com

Not really an adventure, but cool:
http://www.espgame.org/
(This one's about labelling images... read the FAQ :)

Killer - Adventure? Comic?
http://www.submarinechannel.com/channel/content/killer/navigate.htm

Another nice one (like Samorost):
http://questfortherest.com/

How about this:

A Case of the Crabs

http://www.otterarchives.com/bountygame.html

Plus PinHead Games:

http://justadventure.com/IndependentDevs/PinheadGames/index.html

I recommend these two articles for an overview of some of the more popular freeware adventure games currently available:
http://www.adventuregamers.com/underground/display.php?id=339
http://www.adventuregamers.com/underground/display.php?id=203

That, and the excellent web stuff mentioned in the other posts. Samorost has reached almost legendary status.

I've made (and continue to make) a few games of my own.
Some are better than others, some are worthless, depending on who you ask.
http://www.rodekill.com

I'm currently getting ready (as in, learning the necessary information, buying the necessary things, and getting people that will be willing to help) to do an adventure game with all the animations in stopmotion. I'll be using AGS, most likely. Maybe I'll put up a website...

A nice game with good graphics (the first act has already been released and translated into english):
http://www.patrimonium.de

There's a lot of Fan-Adventures here (The Games Page of AGS a freeware Adventure Development Kit):
http://www.agsforums.com/games.php

http://ron.the-underdogs.org/

Open-to-all multi-author shared-universe adventure agmes about happenings in a small town. Varied quality, due to the varying authors, but when it be good, it be very very good.

We make adventure games for kids...
http://www.curiosoft.com

I've created a very powerful role playing game engine, targetted to complete with the Bioware's Infinity Engine. :)

http://www.explore-rpg.com

How about apprentice? http://herculeaneffort.adventuredevelopers.com/apprentice.html
It isn't mine but I highly recommend it

Apprentice 2 (and 1 as well): http://herculeaneffort.adventuredevelopers.com/app2.html#top

5 Days a Stranger and 7 Days a Skeptic: http://www.fullyramblomatic.com/7days/

This website has a pretty comprehensive list: http://www.trumgottist.com/crowsnest/games.php

Ron, will this list be available?

I made one called The Sydney Mystery which is here: http://www.twilightsoftware.com/sydneymystery

Chasm is a brilliant flash adventure from a couple years back, that has some nice varied puzzles, great character design and impressive art.

http://www.abc.net.au/gameon/chasm/

more of the same dude's work at:

http://www.transience.com.au/

http://frictionalgames.com/?q=penumbra
Although it is first person there's no shooting and a lot of puzzles

http://www.phil-reed.com/games.html

"Larry Vales, Traffic Division", an uncomplete trilogy made by the very talented Phil Reed (no, I don't know him). It is really funny. Unfortunately, the guy got tired of making games before finishing the third installment.