Lightrooms latest update (6.6) has a new section in the Develop module called Transform. What's new here is the guided option.
With this option you can straighten horizontal and vertical lines in your image. To show you how it works, I've used one of my old pictures.
Before I go to the Transform section, I checked Lens correction:
Adobe recommends to do this before you use the transformation options. After the lens correction, I opened Transformation and picked Guided.
Just draw lines on those parts of the image that you want to have straightened (horizontally and vertically). Click done to apply it to your image. Now the lines are corrected.
Bear in mind that the result can be that you lose some information from the edges of the picture. This can be easily fixed though by cropping your image.
I find this feature pretty helpful! Especially in a picture like this with many buildings.