For some reason I just encountered this error in Left4Dead 2 – fortunately the fix was fairly straightforward – I just needed to install Left 4 Dead 2 Add-on Support.
Right click on the Steam icon in the system tray and click Library.
Now click on the Tools link.
Look for Left 4 Dead 2 Add-on Support in the list.
If it is not installed then simply Install it. If the install is incomplete, right click it, and then select Delete Local Content and then reinstall it.
Thanks to Goathief on the Steam forums for the fix.