In the crypto space the long tail of projects suffer from lack of devs and there is always a need for new tools. Often the DAOs running the treasury can vote to provide you money to build it.

In the traditional space I had a fairly profitable tool that was for a puzzle-like F2P game (think similar to Words with Friends) that ranked highly on Google. That was ad supported but the hardcore players of the game would use it. I wound up selling it for $5k on an app flipper website when the traffic started to decline as the game got older.