Capcom announced today that Left 4 Dead 2's Coach, Nick, Ellis, and Rochelle will be playable in Resident Evil 6's PC-exclusive Mercenaries No Mercy multiplayer mode, and in the other direction, three of Resident Evil 6's zombies will shuffle into the PC version of Left 4 Dead 2. The crossover, which is being called "the Resident Evil 6 x Left 4 Dead 2 project" (Capcom's idea, we presume) will be free for both games later this spring.
In addition to L4D2's human characters, the Witch and Mini Tank will "make a cameo appearance" in Mercenaries No Mercy, though it isn't clear whether or not the mercenaries will have any mercy. I think they might, but something about it makes me unsure. As for L4D2, Resident Evil 6 offers Lepotitsa, Napad, and Ogroman, which have been "brought to life in Left 4 Dead 2 by Valve’s renowned developers."
In Resident Evil 6, which releases for PC on March 22, the content will "automatically be downloaded in the background" starting April 5th, and the Left 4 Dead 2 content will be available sometime this spring through Steam Workshop.