Not new advice, but it's worth being reminded about. I think programmers actually follow this principle quite a bit. It's just that once they have written a nice tool, they release it as open source software. For a lot of people, I think the message needs to be, "built it for yourself, and try to sell it too".

SlyShy,

Great point! There is a fine line between open source and profitable apps, I just made the call the other day to open source something instead of making a paid app.

Why not sell your newly open sourced app? Lots of companies do it. If not the app you can certainly sell services around it.

Would it be profitable to have advertising in open-source program?

Of course, there would be checkbox to turn them off (or whatever). No recompilation needed.