[CalRobert]

It's hard to fund things where the benefit is diffused among many people. Lighthouses are a classic example of this (you can enjoy their benefit even if you don't pay!). So is public art (why should I pay for public sculptures for everybody else?).

Taxes are a traditional approach to this. It's sloppy and imperfect (plenty of my money is spent on stuff I don't value), but I don't know a much better way.

Similarly, it's hard to price things where the negative externalities are diffused among many people. Pollution is a good example of this - the person generating pollution is often not the one suffering the effects of it, and the effects are generally widespread and diffuse (your car makes a thousand people a little bit worse off, not a single person horribly worse off). Taxes on these externalities are also an appropriate tool here.