Have you run out of space on your C:\ drive?
Or do you have a super-fast SSD that is too small to hold all your steam games at the same time?
Then try Steam Mover.
Following on from a very interesting post on the Steam forums I've created a little app to make moving the files and creating the junction points much much easier!
Here I have my steam folder in the default location, and the application should find that for you. If not, select the \Steam\SteamApps\Common\ folder yourself, and a secondary hard disk
\, where I have created a folder.
Note that Steam Mover can be used to move any folder around (by changing the Source Path), but...