Crop Tool Example
This is an example of how the Crop Tool can be used to straighten an image. Here we have an image that we'd like to adjust with a crop and small rotation.
In the Preview Window, the Crop Tool is available on the right side. Once you set your crop, rotate it by dragging a corner of the crop. There are guidelines to help you find the horizontal or vertical. Note, it is a "non-destructive" crop, meaning that the file is not actually changed. The results are seen when you choose to save the file as a new image.
Once the crop is set, choosing to save the image will result in the crop being applied. He's the result of that crop shown above:
If you hold down the shift key while rotating, it will snap to 45 degree angles, one of which is zero. This can be helpful if you accidentally rotate the crop and need to set it back to level.