1. Open document horizon.jpg
2. Select Measure tool
   With Measure tool draw line following your angled horizon.
3. Image - Rotate Canvas - Arbitrary
   Notice that value is already there and click OK.
4. Use cropping tool to complete operation.
   Note that holding Control key will remove cropping tool magnetism.

In many architectural pictures, vertical lines converge inwards.
Here's how you can correct it.

1. Open documents building.jpg and building2.jpg
2. With cropping tool draw rectangle smaller than image.
3. In options bar check Perspective.
4. Grab the bottom handles (one at a time) and drag them outwards
   so they will be parallel to the vertical lines you are trying to straighten.
   Then use sides and top/bottom handles to maximize selection.
5. Double click inside selection to apply cropping.  Done.