ShiftN - Automatic correction of converging lines
When taking photos of high buidling, your image will have a perpective distortion. For correcting you can use graphic programs like FixFoto. The free software ShiftN can do it automatically.
The website is German, though ShiftN "speaks" English and German.
I've corrected one of my images taken during my holidays and was pretty surprised:
Original:

Entrance of Castle Caernavon (Wales), 2006 Helmut Müller
Correction with ShiftN:

ShiftN has corrected the above image automatically and cropped it to the largest possible area when saving. Thus correction is done easily and quick. Using parameters one can influence the correction and the result. The user interface is not high end, but still easy to use.
Sure enough not a filter for all photographs, but interesting nevertheless.
Greetings, Helmut