You can get many of these games for free here :   http://www.adventuregamestudio.co.uk/site/games/

Other games are here, the demos are free:   http://http://www.wadjeteyegames.com/games/

Here are some great games:

Resonance  

Maniac Mansion Deluxe  

Gemini Rue  

Primordia  

Blackwell Epiphany  

6 Days a Sacrifice  

5 Days a Stranger  

King's Quest II+ VGA  

A Second Face  

The Marionette  

King's Quest I  

Prodigal  

Shifter's Box - Outside In  

Eternally Us  

Infinity String  

Two of a Kind  

Educating Adventures of Girl and Rabbit