When using the Crop tool, moving your cursor outside of the Crop changes the icon to a double headed arrow, clicking and dragging (outside of the crop) will rotate the image (straightening a horizon for example).
Command (Mac) / Control (Win) toggles to the Angle tool while cropping. Using a straight line in the image, for reference click and drag over the reference line (such as a horizon) to straighten.
Cmd+ Opt /Ctrl + Alt + drag with the Crop tool swaps to Angle tool and displays an angled Grid Overlay

The last tip above was new to me, but holy crow, even with an i7 CPU and 9GB RAM (Win 7 x64 / SP1), the grid overlay is INCREDIBLY slow and unresponsive. I drag out a line and let go, and the overlay spins around for another couple of seconds by itself. Sounds neat, but this feature is completely unusable on my system. Is this a bug?
I usually just click on the straighten tool icon and I didn’t know about the shortcut. Thanks for posting this!