Crusader Kings II
is now free on Steam and while it doesn't include any DLC, you can use the DLC of the host when joining a multiplayer game.
Archive.org has 2500 free DOS games.
Most of these are not very interesting, but it does include classics like The Secret of Monkey Island, DOOM, SimCity 2000, Rayman, Star Wars X-Wing, Mortal Kombat, Loom, The Incredible Machines 2 and a whole bunch more.
and the expansion BroodWar were made available for free following the release of the remastered version.
has a list of commercial games available as freeware too.
is available for free too.
You can also search for websites offering abandonware for a selection of old games that aren't being distributed commercially any more (I'm not posting a link as the legality of distributing abandonware is iffy).