I thought developer tools would be the toughest to make money off. Developers always believe there is a free, open source alternative out there somewhere, and if not, someone's sure to start one soon.
Personally, I've paid money for the WebStorm IDE, the PyCharm IDE, Sublime, Dash, Paw, Balsamiq, and more. I've bought ready-made UI components, database drivers, etc., and I'm about to start shelling out $4,000+/year for a commercial Qt license even though I could probably get by with Qt's free LGPL version.
And that's just me as an individual; companies are even more willing to spend money on developer tools.