As free plugins developer since 5 years, I agree with this post and comments. But I would like to emphasize the common confusion between free availability (published as feedback of works with WP) and a commercial product. In this contributive context, it's rare, but I'm happier when a user publishes a notice, offers a translation or simply writes his experience as a manual (or a chapter). When a plugin is downloaded thousands of times, I am invited to make progress in updates while following the evolution of the core of WP. As seen at WordCamps for 4 years, there are more and more professionals but they routinely forget that Free open source plugin does not mean free services.
But would I have imagined five years ago that I was going to exchange with users from all around the world?

Michel