In order to compete with proprietary software, open source projects need to be rhetorically savvy as well as technically savvy. IMHO, firefox has been the most rhetorically savvy open source product out there, and it is no suprise that it has been widely adopted. (Hell, I even convinced my parents to download firefox, and I often have a lot of students who are already firefox users before my class). My sense is that firefox has succeeded in gaining relatively wide market share because:
1) There are several easy to understand reasons that firefox is better than IE (more secure, faster, more options); the firefox website sell the software's benefits in a way that largely avoids technical and OSS jargon.
2) Firefox is very easy to use out of the box (if you can use IE, you can use firefox; you can add plug-ins if you want, but the basic interface is very clean and simple)
3) Spreadfirefox.com has led a smart, viral marketing campaign which has enabled many users to act as promoters (in ways that require little time).

Anyway, if you are interested in seeing how firefox coordinates its grassroots, viral marketing, check out spreadfirefox.com. I think there are many ways that we rhetoricians could help out in this effort--both on our own and in conjunction with students. I also think there is a lot we could learn about firefox's rhetorical strategies that we might be able toapply to offering rhetorical advice/support to other OSS projects.