Office Building has been added to the Workshop: http://gtm.you1.cn/sharedfiles/filedetails/?id=123017887

Check the readme.txt for the map if you don't have any idea what the Workshop is: http://ninetofive-officebuilding.tumblr.com/

Basic idea: You no longer have to manually install maps. You 'subscribe' to a map and it is automatically installed and kept up-to-date for you - and you can do all of this in-game, without having to restart L4D2.