Left 4 Dead 2

In March 2010, Valve announced that it would be bringing the Steam content platform to Mac OS X computers; along with it, Steam will be providing native versions of existing Valve games including Left 4 Dead 2 and its predecessor. The games will support cross-platform play, allowing Mac players to play alongside PC players on the same servers, and will also be part of its Steam Play cross-compatible and Steam Cloud titles, allowing a player who has purchased the game on one platform to download and play it on the other platform for free. The Mac OS X client was released on October 5, 2010.

Question: can you play left 4 dead 2 cross platform
Answer: yes