cyberchief.979: They've got:
Alien Carnage
Arctic Adventure
Bio Menace
Major Stryker
Monuments Of Mars
Pharaoh's Tomb
Stargunner (GOG has this one)
Those have been freeware for quite a while now. You can also find them legally at other sites, like
Classicdosgames (all the green ones are freeware releases).
Epic MegaGames also made several of their old games freeware years ago.
Admittedly, I suppose that the releases on the 3D Realms site are readily set up to run on new systems, which would be neat; and we should probably thank them for not simply making them commercial titles again (Cinemaware, I'm looking at you).
EDIT: Never mind, BillyMaysFan59 was faster than me