Probably the premiere free desktop publishing software application, Scribus is open source
desktop publishing software for Windows, Mac, Linux, and other platforms. It has the features of the pro packages (and perhaps the steeper learning curve too), but it's free.
offers CMYK support, font embedding and sub-setting, RIP-ready PDF creation, EPS import/export, basic drawing tools, and other professional level features. It works in a fashion similar to Adobe InDesign and QuarkXPress with text frames, floating palettes, and pull-down menus -- and without the hefty price tag.
Available as of 2011: Scribus 1.3+ for Mac, Linux, Windows (1.3.9 released in 2010)
Scribus can be overwhelming to those unfamiliar with how high-end desktop publishing software works. It's not quite like consumer desktop publishing packages or word processing software. Look here for the best sources of Scribus tutorials
Most everyone has something good to say about Scribus, but it's not perfect. Get the pros and cons from these reviews: