If the recent Left 4 Dead DLC announcement has renewed your interest in the co-op game on the PC, you might want to squeeze in as much playtime as you can tonight. Tomorrow night, Thursday August 6th, Steam will undergo maintenance at 6PM Pacific.
While you should still have access to your games, you won't be able to play anything utilizing Steam's matchmaking services. The Steam Community, Store, and lobbies will all be offline until 9PM Pacific. At least that's the plan - we all know how these upgrades go.