Real estate. (Ok, not 100% passive, but what is?) You can buy a duplex for not much more than a single family house in many US cities and, thanks to the low interest rates for an owner-occupied mortgage, rent out half and live for free. Unlike tech, you will not have a giant windfall with real estate; but also unlike tech, it's very easy to price your product, find customers, and figure out what your cashflow will be…
There are easier ways to live rent-free, unless, as I say, you have the personal skills to ride herd on grown children or if you are lucky enough to own your property in an area where normal people rent, and you can find good tenants on a regular basis.