It's been a loooooong time since we've seen some DLC for Left 4 Dead 2. Almost 2 years ago, The Sacrifice was the last DLC L4D2 players saw, but back in February 2011, Valve did state they were working on more DLC. That DLC, Cold Stream, is finally arriving on July 24th giving you plenty of time to try and buy back your copy of L4D2 that you sold.
The Cold Stream is a new campaign that has been in beta on the PC for more than a year, and is a community campaign by the same people that made 2 Evil Eyes custom map for L4D2. Along with the Cold Stream campaign will be four of the Left 4 Dead 1 campaigns: Death Toll, Dead Air, Blood Harvest and Crash Course.
Source: Left 4 Dead Blog