https://store.steampowered.com/app/44350/GRID_2/

Free to keep when you get it before 22 May @ 7:00pm

http://store.steampowered.com/app/38400/Fallout_A_Post_Nuclear_Role_Playing_Game/

http://www.indiegamebundles.com/original-fallout-free-steam/

Just add the game to your library and it is yours.

http://www.indiegamebundles.com/outlast-whistleblower-dlc-free-steam/

http://store.steampowered.com/app/305050/Outland/
Free for 48Hrs

http://store.steampowered.com/app/236150/Starpoint_Gemini_2/

This game is free until May 24th at 10AM Pacific Time! Once you add the game it will remain in your account after the free period.