If you are a fan of Zombies, but don't fancy buying the full game, then you could be in for luck, as the Call Of Duty Zombies can be enjoyed for free for Jan 14-21 as per the listing on the MS store.Edit
: Confirmation on the PS4 SideEdit: Link for PS4
Experience the next generation of Zombies with Black Ops Cold War Zombies Free Access.
“Black Ops Cold War Zombies is set to take veterans and newcomers alike on a bold and terrifying journey that expands on an iconic part of the Call of Duty franchise.
“This co-op mode includes new ways to progress, classic Perks and an arsenal of Cold War-era Zombies weaponry that will help survivors dominate the legions of undead.”
Credit to the OP at DealLabs who spotted this earlier
Hope it helps someone.