I have been using Scribus for a while. It is a decent DTP program, capable of most things most people will want to do. Its not quite as capable as Adobe Indesign but I'd say its the equal of anything else I've used, which isn't bad considering it's free.
You may not want to write your whole book in it, but its good if you are writing and formatting articles, or if you want to create a newsletter, flyers, posters or other publicity materials.
I'm running the Windows 64 bit version but it is also available in 32 bit, Apple (Lion or Snow leopard) , or Linux
Open source desktop publishing software