From GOG Community Wiki
This page contains a list of links for demos of GOG games. Some GOG games are sold in packs that contain several games, and some of these demos are located on different web pages thus the need for multiple box links. Most of these demos will require some tinkering to get them to work. If you don't know how to install these demos, simply post a message in the gog forums asking for help and somebody may be able to help you.
Note: Cnet downloads include opt-out adware
Note 2: Mouse over game boxes to see the game demo!
USE AT YOUR OWN RISK