With the rise of free to play games, threads devoted to such games could have a link in there for joining/installing in a flash. Here's an example for Firefall: steam://run/227700 It obviously doesn't parse it and putting it in the URL BB code precedes it with http:// so it doesn't work. It could be also used to link to games directly in Steam in the latest sales/deals thread. People like me aren't logged in to the Steam website so the steam:// address is more useful. Edit: Here's the skinny on the protocol.