There are a couple of journals that have rather generous margins around the text. I prefer to print out two pages per sheet to avoid wasting paper, so it would be nice to be able to remove those margins and increase the size of the text instead.
pdfcrop by Heiko Oberdiek allows you to do just that. It's included with Debian Linux, and "sudo apt-get install texlive-pdfetex" will install it. On other (lesser) Linux distributions you may have to check CPAN or CTAN. Once installed, a straightforward "pdfcrop withmargin.pdf nomargin.pdf" will do the conversion.
Update: A reader points out that Tex Live 2008 includes pdfcrop and is available for both Windows and Linux.
Note that pdfcrop should not be confused with PDFCrop. There's also a patch of pdfcrop called pdfcrop2, but I think that covers the current crop.
And spare a thought for Fermat.
Image credit: B.G. Johnson