This is a sketch of a proposal to create a gui to quickly and easily crop pdf files. The main purpose of this software would be to fix margins of multiple-page scanned documents (such as articles scanned by your favourite university library).
Tools like impose+, psnup, and pdftk do a good job at various pdf manipulation, but cropping is often something that is best done by hand. A FOSS tool to allow a user to do this *quickly* doesn't seem to exist, and would be quite beneficial.
This software should be simple. Generally, you run it from a command line and get a window where you can select cropping areas. Then you should be able to cycle through pages (either you get one at a time, with the ability to only advance by one page at a time, or you have a back and forward button so you can modify your selections).
You could use imagemagick to do all the underlying .. well, image manipulation magic. Most simply, you could do a loop through each page, extract the image, display it with some python library (or even tk..), where you let the user select an area on it, which it then crops using imagemagick and appends to the end of a new pdf file.
In the future, more complexity could be allowed. Zooming and/or scrolling (i.e., general issues of scale) will have to be considered from the start. Eventually, adding rotation functionality would be extremely useful as well.