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)

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 needs to export an A5 document to PDF and then tell Adobe Reader to print it as a booklet.
PDFSaM is a very reliable Java-based tool that provides many options to manipulate PDF files. Especially recommended for pre-press work!
PDF-Shuffler is a small GTK+-based tool that allows quick edits, like merging and splitting PDF files or adding/removing/rotating pages.
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)