"Warped horizon lines and bent skyscrapers are just as
disheartening to the travel photographer as the demonic 'red eye'
effect. Red eye correction is commonplace in software today, but
you don't have to live with distortion either. Free software can
correct lens- and perspective-distortion, turning the fishbowl look
into the landscape as you remember it. And even if you don't have
vacation pictures that need a new perspective, the same technique
can come in handy elsewhere--such as for extracting undistorted
textures from a photo to use in a game or 3-D model. Here's how to
"Most people never bother with fixing perspective distortion in
their photos, unless they find a particularly egregious one--and
then they try the pull-on-the-corners Perspective tool in Photoshop
or the GIMP, often with miserable results..."