Exactly what it says on the tin: PoDoFo book imposition plans by Diversiforge.
These are plan files only: scripts which tell PoDoFo how to impose a given PDF. PoDoFo is available from http://podofo.sourceforge.net/ .
podofoimpose source.pdf destination.pdf name-of-plan-file.plan lua
You can print the imposed PDF using the double-sided printing method of your choice.
16 page, 4 sheet booklet (2-up)
This will turn a PDF of single pages into a PDF imposed to four pages to a sheet, four sheets to a signature/folio. I typically use this with a 5.5"x8.5" PDF, to print on 11"x8.5" paper, but any PDF that is half the size of your target sheet size should do.
You should be able to accomplish the same effect with Adobe Reader's booklet printing option, but I haven't tried it.
Tiny book (16-up)
Want a tiny library filled with tiny books? Of course you do. This will cram 32 PDF pages on a single sheet (16 pages to a side). I use this with 11"x8.5" paper and a 2.125"x2.75" PDF, but if you've got bigger (or smaller!) paper, just divide the dimensions by 4 to get your PDF size.
This plan will take a PDF and split it in half vertically. I've used this to make PDF scans of game manuals more usable with an e-reader, but it could also be a useful preprocessing step if you wish to impose a PDF that already has two pages per PDF page.