User SOhammy on reddit compiled this list. He will probably keep updating it as he finds more. Check the thread here:
http://www.reddit.com/r/AskReddit/c..._free_pc_game_that_you_think_everyone/cbvj6qc
Wolfenstein Enemy Territory Wolfenstein Enemy Territory
Cave Story Cave Story
Barkley, Shut Up and Jam: Gaiden Barkley, Shut Up and Jam: Gaiden
Enviro-Bear 2000 Enviro-Bear 2000
Elder Scrolls Games
Elder Scrolls Arena From Bethesda
Elder Scrolls Daggerfall From Bethesda
Elder Scrolls Arena by UESP
Elder Scrolls Daggerfall by Ghost
Quake Live Quake Live
Trackmania Forever Trackmania
Tower of Heaven Tower of Heaven
Cat Planet Cat Planet
Spelunky Spelunky
Abobo's Big Adventure Abobo's Big Adventure
Dwarf Fortress Dwarf Fortress
Facade Facade
I Wanna Be the Guy I Wanna Be the Guy
Merry Gear Solid: Secret Santa Merry Gear Solid: Secret Santa
Moonbase Alpha Moonbase Alpha
Robot Unicorn Attack Robot Unicorn Attack
Robot Vacuum Simulator 2013 Robot Vacuum Simulator 2013
Zork Zork
Abandonware In General (Great Resource) Abandonia
War Thunder War Thunder
Toribash Toribash
Command And Conquer Command And Conquer
Command and Conquer: Tiberium Sun Command and Conquer: Tiberium Sun
Red Alert 1 Red Alert 1
Star Control 2 Star Control 2
Space Station 13 Space Station 13
Path of Exile Path of Exile
Osu! Osu!
Nethack Nethack
Battle for Wesnoth Battle for Wesnoth
Open Transport Tycoon Deluxe Open Transport Tycoon Deluxe
Mari0 Mari0
Perspective Perspective
Minesweeper Minesweeper
Warframe Warframe
Grand Theft Auto 1,2, and Wild Metal Grand Theft Auto 1,2, and Wild Metal
Planetside 2 Planetside 2
Hawken Hawken
Katawa Shoujo Katawa Shoujo
Brogue Brogue
Brogue (tiled Oryx version)
DoomRL DoomRL
Dungeon Crawl Stone Soup Dungeon Crawl Stone Soup
Tales Of Maj'Eyal: Age Of Ascendancy Tales Of Maj'Eyal: Age Of Ascendancy
Unlok Unlok
Battlefield 1942 Battlefield 1942
Urban Terror Urban Terror
Team Fortress 2 Team Fortress 2
Cookie Clicker Cookie Clicker
Blacklight: Retribution Blacklick: Retribution
Iji Iji
Ace of Spades Ace of Spades
Club Penguin Club Penguin
MechWarrior Online MechWarrior Online
Realm of the Mad God Realm of the Mad God
World of Tanks World of Tanks
Digital: A Love Story Digital: A Love Story
JFK: reloaded JFK: reloaded)
Black Mesa Black Mesa
Dink Smallwood Dink Smallwood
Second Life Second Life
Desktop Dungeons Desktop Dungeons
Firefall FireFall
Warzone 2100 Warzone 2100
Dota 2 Dota 2
Alien Swarm Alien Swarm
Super Crate Box Super Crate Box
The Kingdom of Loathing The Kingdom of Loathing
Synergy Synergy
Ghost Recon Online Ghost Recon Online
Sauerbraten Sauerbraten
APB Reloaded APB Reloaded
Yume Nikki Yume Nikki
Smite Smite
Tribes:Ascend Tribes:Ascend
Arm Joe Arm Joe
Knytt Knytt
Knytt Stories Knytt Stories
League of Legends League of Legends
Runescape Runescape
Rift Rift
TERA TERA
SkiFree SkiFree
Nitronic Rush Nitronic Rush
Xonotic Xonotic
Open Arena Open Arena
Warsow WarSow
http://www.reddit.com/r/AskReddit/c..._free_pc_game_that_you_think_everyone/cbvj6qc
Wolfenstein Enemy Territory Wolfenstein Enemy Territory
Cave Story Cave Story
Barkley, Shut Up and Jam: Gaiden Barkley, Shut Up and Jam: Gaiden
Enviro-Bear 2000 Enviro-Bear 2000
Elder Scrolls Games
Elder Scrolls Arena From Bethesda
Elder Scrolls Daggerfall From Bethesda
Elder Scrolls Arena by UESP
Elder Scrolls Daggerfall by Ghost
Quake Live Quake Live
Trackmania Forever Trackmania
Tower of Heaven Tower of Heaven
Cat Planet Cat Planet
Spelunky Spelunky
Abobo's Big Adventure Abobo's Big Adventure
Dwarf Fortress Dwarf Fortress
Facade Facade
I Wanna Be the Guy I Wanna Be the Guy
Merry Gear Solid: Secret Santa Merry Gear Solid: Secret Santa
Moonbase Alpha Moonbase Alpha
Robot Unicorn Attack Robot Unicorn Attack
Robot Vacuum Simulator 2013 Robot Vacuum Simulator 2013
Zork Zork
Abandonware In General (Great Resource) Abandonia
War Thunder War Thunder
Toribash Toribash
Command And Conquer Command And Conquer
Command and Conquer: Tiberium Sun Command and Conquer: Tiberium Sun
Red Alert 1 Red Alert 1
Star Control 2 Star Control 2
Space Station 13 Space Station 13
Path of Exile Path of Exile
Osu! Osu!
Nethack Nethack
Battle for Wesnoth Battle for Wesnoth
Open Transport Tycoon Deluxe Open Transport Tycoon Deluxe
Mari0 Mari0
Perspective Perspective
Minesweeper Minesweeper
Warframe Warframe
Grand Theft Auto 1,2, and Wild Metal Grand Theft Auto 1,2, and Wild Metal
Planetside 2 Planetside 2
Hawken Hawken
Katawa Shoujo Katawa Shoujo
Brogue Brogue
Brogue (tiled Oryx version)
DoomRL DoomRL
Dungeon Crawl Stone Soup Dungeon Crawl Stone Soup
Tales Of Maj'Eyal: Age Of Ascendancy Tales Of Maj'Eyal: Age Of Ascendancy
Unlok Unlok
Battlefield 1942 Battlefield 1942
Urban Terror Urban Terror
Team Fortress 2 Team Fortress 2
Cookie Clicker Cookie Clicker
Blacklight: Retribution Blacklick: Retribution
Iji Iji
Ace of Spades Ace of Spades
Club Penguin Club Penguin
MechWarrior Online MechWarrior Online
Realm of the Mad God Realm of the Mad God
World of Tanks World of Tanks
Digital: A Love Story Digital: A Love Story
JFK: reloaded JFK: reloaded)
Black Mesa Black Mesa
Dink Smallwood Dink Smallwood
Second Life Second Life
Desktop Dungeons Desktop Dungeons
Firefall FireFall
Warzone 2100 Warzone 2100
Dota 2 Dota 2
Alien Swarm Alien Swarm
Super Crate Box Super Crate Box
The Kingdom of Loathing The Kingdom of Loathing
Synergy Synergy
Ghost Recon Online Ghost Recon Online
Sauerbraten Sauerbraten
APB Reloaded APB Reloaded
Yume Nikki Yume Nikki
Smite Smite
Tribes:Ascend Tribes:Ascend
Arm Joe Arm Joe
Knytt Knytt
Knytt Stories Knytt Stories
League of Legends League of Legends
Runescape Runescape
Rift Rift
TERA TERA
SkiFree SkiFree
Nitronic Rush Nitronic Rush
Xonotic Xonotic
Open Arena Open Arena
Warsow WarSow