Back on March 15 it was announced that the PC version of Resident Evil 6 was to do a cross over with the very popular Left 4 Dead 2. The crossover would entail that zombies from RE6 would be making an appearance in L4D2, while the survivors from L4D2 were to be playable characters in RE6's Mercenaries: No Mercy mode. Today Capcom announced that the crossover is available now and that fans can download the cross over characters and zombies.
For those who forgot about this crossover, here's the trailer that was used to revealed it back in March:
Again the Resident Evil 6 x Left 4 Dead 2 is now availabe for download on Steam.
Source: Capcom Unity