Maybe it shouldn't be, given the subject matter, but this is a pretty big surprise. Capcom and Valve sent out a press release earlier announcing a collaboration in the form of Resident Evil 6 x Left 4 Dead 2, which is coming to PC as a free update.
Owners of the PC version of Resident Evil 6 will be able to play as one of the four survivors from Left 4 Dead 2—Coach, Nick, Ellis, and Rochelle—in the PC-exclusive The Mercenaries No Mercy mode. The update also adds two of L4D2's Special Infected zombies, the Witch and Mini Tank, to the mode. The new content will automatically download in the background of the PC version starting on April 5.
Likewise, Steam Workshop tools were used to bring some of Resident Evil 6's enemies into the PC version of Left 4 Dead 2. The Lepotitsa, Napad, and Ogroman will join the zombie hordes of Valve's game this spring.
Over at Capcom-Unity, a call has been made to get users cracking on some L4D2 versions of Leon, Jake, Sherry, and other key RE6 cast members using the Workshop tools.
Finally, here's a developer diary from Resident Evil 6 producer Yoshiaki Hirabayashi: