I had to straighten a pic today, and it occurred to me that not everyone knows the trick.
Thanks to my beautiful wife for this one!
Just choose your Measure Tool (probably hidden under the Eye Dropper Tool) and draw a rule along the horizon (or another desired horizontal or vertical line in the photo).
Then go to Image > Rotate Canvas > Arbitrary. Voila! You'll find the angle of the rule is automatically entered there, so all you need to do is hit Ok, and the photo rotates to the perfect angle!
The act of rotation will naturally add some blank triangles into the corners of your image. So you can go ahead and add a blank layer, choose your Clone Tool, make sure it's set to "Sample: Current & Below" or "Sample: All Layers" in the Options Bar, then clone new detail into those corners if you wish. (The Content Aware Fill function, if your version has it, can also work sometimes).