[mithrandir]

I came upon this freeware for correcting keystoning of architectural photos:

Marcus Hebel

Go to Photografe and look for ShiftN

[troyz]

Hugin will also do perspective correction!

Hugin tutorial — Perspective correction

Procedure is:

-- input your image
-- set the output projection to "rectilinear"
-- input a few horizontal and vertical lines

Hugin will solve for the yaw, pitch, and roll of the camera and transform the image accordingly.