I work at a small, non-tech family business. I am the default IT guy, and I have made a ton of internal tools for us. Most of these are either only useful to us or would be hard to make universally for others. However, I have turned a few into B2B web apps that have good traffic and earn a couple hundred each month with virtually no effort or optimization. And the opportunity exists to turn these into more robust SAAS offerings or do contract work, but that would be less passive.

Not everyone is in this position, so it is a very interesting conundrum. If you know anyone that works for or owns a small business or works as a professional in a specific field, ask them about the tools they already use and what tools they would like to have. Tell them to let you know the next time they are facing a problem that they know could be done better/easier/faster through a tool or app. If you can make something that will help them, perhaps it will help others. Custom forms, data collection, etc.

It requires talking to people who have these problems ;) Most programmers only end up talking amongst themselves, or attack business problems they themselves experience (hence the large market saturation for developer tools).

Listen to what problems people have. I had lunch with a friend today that has a niche need. He owns a 2 truck garbage truck company. He has a few hundred customers, and he wanted some way to document the route. He tried using Google maps but it didn't work easily enough, the industry specific software that is already out there is sized for 50 truck fleets not 2 truck fleets, etc.

If anyone has already built that app, please let me know.

An easy way is to look for a company willing to solve a specific problem that may exists in similar companies. If you make the right contracts you can replicate the solution later.