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3D Realms has seven DRM-Free games available at their site.
The games are:
Alien Carnage
Arctic Adventure
Bio Menace
Major Stryker
Monuments of Mars
Pharaoh’s Tomb
Stargunner
You need to register for a 3DR account and verify your email address.
The free games get added to your library automatically. You can see more info about the games on the games page.