Microsoft's "Netflix for video games" is starting to look like a better deal all the time as the catalog of titles available to play continues to grow. As of today, October 3, there are 137 games available to download and play, and of those, 48 are co-op games.
The way Xbox Game Pass works is that for a monthly subscription fee ($9.99) you are free to download/play any of the titles available via the program. If you let your subscription expire, you will no longer be able to play the games (unless you already own them, that is) until you renew your subscription. You find out more on the official site, which also includes a list of all title available (there's also this hand searchable list on Eurogamer).
Here are all the co-op titles currently being offered via Xbox Game Pass. We'll do our best to update this list as more titles are added.A Kingdom for Keflings (Xbox Live) A World of Keflings (Xbox Live) Age of Booty (Xbox Live) Bionic Commando Rearmed 2 (Xbox Live) Borderlands (Xbox 360) Capcom Arcade Cabinet (Xbox Live) CastleStorm (Xbox Live) De Blob 2 (Xbox 360) Dead Island Definitive Edition (Xbox One) Dead Rising 3 (Xbox One) Defense Grid 2 (Xbox One Double Dragon Neon (Xbox Live) Dungeons & Dragons: Chronicles of Mystara (Xbox Live) Fable II (Xbox 360) Fable 3 (Xbox 360) Farming Simulator 15 (Xbox One) Final Fight: Double Impact (Xbox Live) Gears of War (Xbox 360) Gears of War 2 (Xbox 360) Gears of War 3 (Xbox 360) Gears of War: Judgment (Xbox 360) Gears of War: Ultimate Edition (Xbox One) Guacamelee! Super Turbo Championship Edition (Xbox One) Halo 5: Guardians (Xbox One) Halo: Spartan Assault (Xbox One) Iron Brigade (Xbox Live) Kameo (Xbox 360) LEGO Batman: The Videogame (Xbox 360) Metal Slug XX (Xbox Live Arcade) Metal Slug 3 (Xbox Live) Monaco: What's Yours is Mine (Xbox 360) Ms. Splosion Man (Xbox Live) Payday 2: Crimewave Edition (Xbox One) Perfect Dark Zero (Xbox 360) Resident Evil 6 (Xbox One) Sacred 3 (Xbox 360) Saints Row: Gat Out of Hell (Xbox One) Saints Row IV: Re-Elected (Xbox One) SEGA Vintage Collection: Golden Axe (Xbox Live) SEGA Vintage Collection: Streets of Rage (Xbox Live) Splosion Man (Xbox Live) Sunset Overdrive (Xbox One) Super Mega Baseball: Extra Innings (Xbox One) Terraria (Xbox One) The Bug Butcher (Xbox One) Toy Soldiers: Cold War (Xbox Live) Viva Pinata (Xbox 360) Vita Pinata: Trouble in Paradise (Xbox 360)