On a Windows machine, the Steam application allows you to relocate the “SteamApps” folder to a different drive. This allows you to effectively shift your content around anywhere you like. When you install a new game, it’ll even ask you where you want to install it.
For some bizarre reason they did not afford the same convenience to Mac users. Initially when Steam was released for the Mac they even forced you to keep the SteamApps folder in your Documents folder. This made it really painful to use a portable home directory. Valve relented after a huge user outcry and relocated the folder to ~/Library/Application Support/Steam, which is probably where it belonged anyway.
