While not a big commercial success like the rest of the open source projects on this list, I chose Audacity because personally I love using it and find it very useful. I am not aware of any commercial company offering support and services for Audacity, but who needs it? It just works. If you have to work with audio files, Audacity sets the bar for ease of use and powerful tools. It has been downloaded something like 70 million times, so that should say something.