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!