With its most recent Steam client update, Valve has added the ability to move the install folders for your Steam games. Before this update, you had to either install a third-party app or use a lengthy workaround to move the install folders for your games if you wanted to avoid re-downloading games to a new hard drive. Now, this ability is but a click away.
As Artem Russakovskii points on on Google+, Steam offers a button to move install folders. Here's where to find it:
1. Go to your Steam Library, right-click on a game and select Properties.
2. Click on the Local Files tab and you should see a button for Move Install Folder.
I received the update but still don't see the button. Allow me, however, to offer you a mockup of what it'll look like as I wait for it to roll out to me:
Unfortunately, there appears to be no way to select multiple games to move as a batch. Instead, you must go game by game to move your install folders.