One of the "hidden" features of the Lens Correction filter is a straighten tool. Okay, it's not really hidden, but most people don't notice it when they're using the filter to adjust vignetting etc. The straighten tool couldn't be simpler: just draq across what should be straight, and it straightens the image. Then use the zoom control to enlarge the image just enough that you don't even have to crop it. Sweet!
Kudos to Matt K for "discovering" that one!