Overkill Software have announced a new project with Valve, combining their cops 'n' robbers co-op game with Valve's own co-op zombie shooter:
The crossover began as nods to the Left 4 Dead universe were added in a patch for Payday, such as a pile of Left 4 Dead-style medkits. Payday players quickly noticed the changes. We don't have any further details on what this "blend" of the games will be, but we'll keep you updated as details emerge.
You can buy PAYDAY: They Heist from Steam here and read the original blog post here.