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Well, I certainly can name one piece of open source software that seems to be almost guaranteed break a patent of Microsoft.
It is not a too big difference. M$ have about 10 000 patents or more. If they want sue any FOSS (or any other) project, probably they can. And there is a Novell behind the mono with their own patents. It is also true for java and SUN, but who is behind the gcc, KDE, GNOME, PHP, Python, SAMBA, Perl, Apache, wine, etc ? And it is also true for smaller companies and their products: Qt is also can hurts any patent. Or your own commercial application...
If you don't know how many and what kind of patents have the microsoft you never will in safe with any FOSS product. The software patents are generally bad things, it is very good for big software giants like M$, IBM, SUN, etc but can kill any smaller company or project.