Bradley Kuhn Executive Director of the Software Freedom Conservancy, wrote an interesting perspective piece on open core in 'Does “Open Core” Actually Differ from Proprietary Relicensing?' If you have never read anything from any of the followers of RMS(Richard Stallman) this is an interesting example. While I appreciate the points there is a distinct lack of reality in what is written. I love freedom and promote Linux every chance I get. I want to see it and other free and open source software over take the closed proprietary software. However, there is a distinct hatred for anyone making money on Free and Open Source software from almost everyone RMS has converted. The reality is though that people can't support a family without coming up with multiple different was of making money on the products in the Free and Open Source world. I don't see how selling add-on products, freemium models, and support means that you can't be an open source promoter? The main point of the article seems to be that making money on Open Source is a bad thing unless it's just for support. The article does link to some attempts to define the newer "Open Core" with one of the best ones here.