Amnesia: The Dark Descent is free on Steam right now: http://store.steampowered.com/app/57300/

As far as I can tell, it's a permanent free (like "On the House" for Origin).

Speaking of Origin's On The House!

https://www.origin.com/en-us/store/buy/c-c-the-ultimate-collection/pc-download/base-game/standard-edition

Command & Conquer Red Alert™ 2 and Yuri’s Revenge™ are both free at the moment (Not the rest of the C&C collection)

Just as an FYI, the last two On The House games lasted for 70 days each.

If this pattern continues, the next OTH game will be released November 24th, 2015.

Apparently, the demand for the C&C games are so high it's been temporarily removed, and at least according to the Origin client, you can get Theme Hospital for free if you don't already have it.

Edit: Whatever was causing that is fixed now, and they're offering the C&C games again. No Music Factory is involved.

Funny you should mention that, the C&C offer just went back up instead of the Theme Hospital placeholder!

Heh, you got your message in just 27 seconds ahead of my edit. :)

https://www.indiegala.com/store#giveaway

"Burgers" is currently free. I think I would have preferred actual burgers.

8 Bit Boy is being given away by PC Gamer and Bundle Stars here: www.pcgamer.com/have-a-free-steam-key-for-retro-platformer-8-bit-boy/

You have to like some FB pages.

Oddworld: Abe's Oddysee® is currently free on Steam for the next 24 hours

Super Distro! is free now.

https://www.indiegala.com/free2play#giveaway

http://www.pcgamer.com/grab-a-free-steam-key-for-time-gentlemen-please-and-ben-there-dan-that/

A bit late on this one since I just got back from travelling, but Anoxemia is free on indiegala at the moment:

https://www.indiegala.com/store#banner_form_box

http://www.pcgamer.com/grab-a-free-steam-key-for-tactical-mech-game-bionic-dues/

Platypus 2 is free right now

https://www.indiegala.com/free2play#banner_form_box?p2

I couldn't get the Bionic Dues thing to work. I tried doing all the social media junk, but nothing :P Maybe the email is just delayed, but it seems like the button isn't even enabled. It looks kinda neat, but I'd probably never play it anyways, so I guess it doesn't matter hehe.

All I ever did to make the pcgamer ones work was like their facebook page. Never bothered with any of the other stuff listed and it's given me keys without fail since then.

Another freebie

http://blunt.itch.io/drones-blast-arenas

If you missed the giveaways for Skyborn; 8-Bit Boy; Ben There, Dan That & Time Gentlemen, Please; and/or Bionic Dues, there's a 2nd chance at each here:

http://www.pcgamer.com/free-steam-key-encore-giveaway-five-games-up-for-grabs/

Theme Hospital is free (again)

https://www.origin.com/en-us/store/buy/theme-hospital-origin/pc-download/base-game/standard-edition

Blood of Old and Quantum Rush: Champions. You have to sign up for a newsletter, join some Steam Groups and visit some groups on Facebook, though apparently you don't have to like the groups in question.

https://www.ushockgaming.com/free-game/

Vertiginous Golf is free for 24 hours on Steam. It's one of those deals where if you add it to your library, you can keep it (just like Origin's "On the House").

http://store.steampowered.com/app/272890

Thanks @moho_00! I got it!

https://www.indiegala.com/store#banner_form_box

Vertical Drop Heroes HD free again in case it was missed last go around

Nice! I've been interested in Vertical Drop Heroes for a while, thanks!

http://www.tribesuniverse.com/

Like the old Tribes games? Alllll here (I think Starsiege: Tribes is missing. It's listed as such, but it's just the Starsiege disc)

Bear in mind that Tribes: Aerial Assault is a ps2 .iso file, so you'll need an emulator (or modified console to play a burned disc)