Everything can be sold if you can find the right buyer. When it comes to browsers Google is the customer, and users are the product. When doing free open source software, your product can not be the software itself. People don't pay for software, they pay for solutions. What problems are your software solving ? How can you package and sell that solution ? People less smart then you are packaging water and selling it in bottles. A lot have to do with branding.

When going from free to paid, consider the probability that the only reason people use your service is because it's much cheaper then the alternatives.