Scribus is an open-source program that brings professional page layout to Linux/Unix, MacOS X, OS/2 and Windows. Scribus supports professional features, such as CMYK color, spot color, separations, ICC color and robust commercial grade PDF.
I have used Scribus 1.4.x for Windows (7-64) on a very powerful laptop fairly often for a technical book. The learning curve is moderate-to-difficult but any DTP software is very complex. It has quite a few inconsistencies and eccentricities. For one example, to change values, occasionally the spin arrow buttons might work but direct numerical entry into the value box may not work. These little things are sporadic but create minor annoyances. It requires greater patience than a commercial release. I had originally written here that Scribus 1.4.x crashed once or more per session (my sessions tend to be very long). After a recent crash the Win7 Action Center function reported that it had adjusted Scribus 1.4.4 to run in "Compatibility Mode." This was done automatically by Win7 and has reduced the crashes greatly to the point where I no longer worry as much but still save often. It was only after many, many dozens of crashes and quite a few 1.4 versions that Windows finally made this automatic adjustment. Others will want to experiment with "Compatibility Mode" to attempt to make this adjustment manually immediately after install. Another reviewer here complained that the panels (panel docking and collapsing) need serious re-engineering to become useful. I suspect that reviewer had not incorporated the use of "F" keys into his workflow. With a dual display setup I drag all panels to their own display. Then just press the panel's F key twice and the panel rises to the top so it works well enough on dual displays. My complaint is that once a panel is closed completely it does not remember the position I had previously assigned to it and I must reposition it every time it is newly reopened. Given that it's free it's just good enough for me to want to continue to use it, working through the occasional frustrations. Part of that is not wanting to spend for InDesign much less learn a new workflow, and knowing that Publisher is not nearly powerful enough. The development isn't perfect but will likely continue to improve. The official online user forum is good enough for finding answers. With all that said, if you have ample patience, you can make very beautiful and professional documents with Scribus. The majority of Scribus is sound and robust. It can do most anything that the commercial products do and a few things they cannot.