Apple's "Save as PDF" feature allows you to create PDF files from any application which can print. However, it doesn't respect the Layout settings. If you want to put two or three (or more) pages on a single page, you're out of luck.
The PDF Nup Maker makes creating N-up PDF files as easy as drag-and-drop. Pick the settings you want, drag a PDF file to the preview window, drop, and wait a few seconds. A new PDF will appear in the preview window, which you can then drag to the appropriate folder in the Finder.
You can download the PDF Nup Maker below. In order for it to work, you first need to have a working LaTeX distribution. A well-known, and widely used, distribution for Mac OS X is maintained by Gerben Wierda and can be downloaded from www.rna.nl/tex.html.
Once you have a working distribution of TeX, download the following:
PDF Nup Maker (Version 1.0)