Straightening Based on a Vertical surface
Camera Raw has a Straighten tool to straighten your photos. What a lot of people don't know is that if you don't have a perfectly horizontal surface, such as a horizon line, to straighten your photos with, you can click-and-drag along any vertical surface. See, horizontal surfaces aren't always perfectly horizontal because of perspective. However, a vertical surface is always vertical, so it's a good guideline to go by.