The 4.3% of Steam users who aren't on Windows aren't out of luck.
Got six games to move? Don't want to wait for each one to finish and start the next one manually? It's your lucky day.
Move all your favorites to an SSD? Done!
Applications like Steam Mover and SteamTool were created to move games from one drive to another while leaving links to them in the original locations so that Steam would still have access to them. In 2012, Valve added support for installing games to different directories to Steam in this update. Steam Mover and other applications were never updated to be aware of this.
Relief Valve is (to my knowledge) the first application that works with Steam's multiple library support.