What would it take to get developers to pay for something that is already freely available as open source?

Question

For example, I know that open-source versions of IL readers/writers exist such as Cecil, and Microsoft's (closed source) CCI. What would it take to get developers to pay for something that is already freely available?

I have always wanted to start my own ISV by writing my own tools and selling them in the open market, but it's hard to gauge the demand, given that free alternatives already exist. Does anyone else have any successful experiences selling a commercial version of an open-source product?