Photographing high buildings gives you a picture where the left and right sides converge toward the top, making it look like the building is topling over backwards any moment.

Before digital photography, the only way to fix this were shift lenses. With digital images, you can fix the distortion, but that used to be a partly manual process.

Enter ShiftN. With the click of one button, it looks for lines which should be vertical, and modifies the picture to look like it was taken with a shift lens. Here is an example picture I took of the Bibliothèque Nationale in Paris. Left is the original, right the version ShiftN came up with.

ShiftN is freeware. It is written for MS Windows, but runs perfectly on Linux, thanks to Wine.

And although the ShiftN Web site is in German, the program's UI comes with English texts.