I'm going to use a pic thats kind of extreme as an example, but this works on any picture. Sometimes there is a vertical line or a horizontal line in the picture that proves the camera angle was off and you would like to have it straight.
Here is a photo of a photo.
In your tool box, the eyedropper is usually open by default.
Hold shift and tap the "I" key till you have the measure tool.
With the measure tool, click and drag along a line in the pic you want either horizontal or vertical.
Go to Image, Rotate Canvas, Arbitrary. When the Dialog opens it will already have the rotation angle written in as well as the rotation direction. Click OK.
Crop and go!!
This is amongst my most useful tools. Hope it helps!
Bad Guys II
Linear Mode
