BookLightning for Mac

From Metaobject:

BookLightning is a Mac OS X 10.5 native utility for imposing PDF files into booklets, either for further processing or to print directly on a duplex or non-duplex printer. Operation couldn't be simpler: just drop a PDF file on the BookLightning icon, and it will create a new PDF with the pages of the original file arranged on sheets ready for folding and stapling. BookLightning does its job very quickly, generating the new PDF file typically takes less than a second.

In order to create a booklet that will be created by printing on and folding letter size paper ( 8.5 x 11 ), set up your original document's page size to half that size: 5.5 x 8.5. This is exactly the size that the individual pages will appear at in the final booklet, because two of them have to fit on each side of the letter size sheets. For users with ISO "A"-style paper sizes, just drop down one size from the size of the paper you will be printing on.