Coincidentally I tried to find a bunch of freeware games not too long ago
Tribes 1, 2 and Ascend are all available for free
GoG is worth signing up for (Ultima IV, Beneath a Steel Sky and others are on there free of charge. They also occasionally have additional freebies for a limited time, like the Fallout giveaway that happened recently)
If your conscience will let you, you can collect indie bundles for a cent or a dollar (depending the bundle). It's technically free but it's worth metnioning
Tag: The Power of Paint is free and it's basically the repulsion and acceleration gel puzzles from Portal (the people who made it actually got hired by Valve and worked on Portal 2. Valve in action)
I'll second the Dwarf Fortress suggestion as well. It's definitely worth checking out if the interface and graphics dont turn you away. Same goes for Nethack
I'm also going to have to second Realm of the Mad God, SMNC, Spelunky, Cave Story and Alien Swarm
Xonotic is great but needs more people
I'm pretty sure most of the old Mechwarrior games are freeware, as well as TES: Daggerfall
If this were 2010 I'd tentatively suggest Global Agenda, but at this point not even I can have fun with it. It's also fairly niche and the netcode isn't very good. Absolutely adore the community there though (on the forums moreso than say... in AvA)
For MMO's I liked DnD Online, DC Universe and Champions
Bloodline Champions is fairly amusing
If you have Half-Life 1 or 2 you're pretty much set. There's roughly a bajillion (totally official number, do not doubt) mods, both single and multiplayer