Introduction

First: you may not know what imposition is: Wikipedia will tell you.

A list of tools which are useful for imposition.

Two type of software are listed here:

software which can be obtained free of charge and can help you do some imposition tasks.
imposition software (free, free of charge or payware)

There is also another wiki page on just Imposition.

Multiplatform

example for making a booklet:

$ java.exe -cp multivalent.jar" tool.pdf.Impose -dim 2x1 -layout 2,3,4,1
-paper "297x210 mm" -verbose file.pdf

PoDoFo Impose (free; imposition tool distributed with the PoDoFo library)
On-line imposition with PoDoFo (web based, work in progress; by Pierre Marchand; there is still an old server)
Adobe Reader 8 and 9 will do booklet printing for you. For a A4 booklet, you just need to export an A5 document to PDF and then tell Adobe Reader to print it as a booklet. With Reader 9 you do not have to prepare a certain size as the Reader can scale (not sure if any precision issues). Depending on needs you may want the "Multiple Pages Per Sheet" or the "Booklet Printing" option in the Print Dialog "page scaling" option drop down.
PDFSaM is a very reliable Java-based tool that provides many options to manipulate PDF files. Especially recommended for pre-press work!
jPDF Tweak is a versatile Java-based tool with loads of options to edit PDF files.
PDFedit is a collection of Qt3 scripts assembled under a GUI. It permits editing PDF files down to the level of text editing, font changes and more.

pstops (see How to make impositions with pstops)
pdf-shuffler (introduction by bigbrovar) Not really an imposition program, but permits easy reshuffling and adding/deleting of pages.

PDFmergeX (donationware): not really an imposition tool... but it may have enough features for you
CombinePDFs (donationware)