Scribus is a desktop publishing program capable of professional quality output. Being Open Source, it is available on any platform that any developer cares to compile it for. In reality, this means that their are official releases for Linux, Mac OS X and Windows.
Desktop publishing is not the sort of task you just jump in and do it, whatever the program. Scribus has a lot of tools and options to give precise control over the layout of a document, so a good manual is necessary to get the most out of it. Scribus is well documented, both in the accompanying online manual and the Scribus Wiki. The latter has lots of practical advice, HOWTOs, tips and tricks contributed by the Scribus Community.
Laying out a document is only part of the job, you need to be able to export the result in a format suitable for printing, and Scribus has plenty of choices here. Whether you want to run off a newsletter on your own printer or prepare a document for professional printing, Scribus is an excellent tool for the job, but be prepared top do some learning to get the best out of it. The latest release, 1.3.6, is classed as a development release but has proved stable and reliable in general use.