There are lot of places on the net which keep free or "abandonware"
titles. Those are games which are abandoned and not selled by their
manifactures or explicitly transformed into freeware. Here are some
links to so called 3rd person classical adventures...
You can also find remakes of adventures. Such remakes of old Lucas Arts
adventures you can download on AGS site: Maniac
Mansion Deluxe & The
New Adventures of Zack McKracken
(Those were made by Lucasfan
Games, which are taken off-line currently. The reasons are not
clear). Kings Quest 2+ VGA
(remake of Sierra old title)
All of those games and lots more use a free engine and development
enviroment called AGS (by
Chris Jones) [seems to be down at the moment I'm writing this]. At AGS
site you can find a lot more original free adventures. There are
several of them which are interesting to play, like "5 days a
stranger", "Apprentice 2", "Jessica Plunkenstein", etc...
ScummVM
Beneath a Steel Sky +
Flight of an Amazon Queen + ScummVM
Those three are separate downloads hosted on same page. The
first two are adventure games which are best played within the SCUMMVM
interpretter. It is a windows interpretter for lot of old DOS adventure
games, to use it just download and install it, and then use the add
game to point the interpretter to a directory holding the files of your
game. That way you don't have to worry about compatibility with your
Windows version, mouse and sound drivers and so on...
You might also try to search the net for instalations of other games
which can be played with ScummVM as Lucas Arts' Indiana Jones titles
(Fate of Atlantis, Last Crusade), Loom and
Monkey Island serries. Check this abandonware archive for
those.