[emmett]

What is the largest and most successful software tool? How much money does it make? [1]

You can sell dev tools, but the market is small and fairly stingy. There are also lots of good free opensource alternatives, because your users are perfectly capable of building and maintaining their own tools.

[1] I believe the answer is probably Visual Studio, which as far as I know is run as a loss leader to promote Windows. Given the amount of development effort going into it, I doubt they sell enough copies to cover the cost.