You might find a lot of insight in Chris Anderson's Wired article Free! Why $0.00 Is the Future of Business.
You will however find many examples where the developers accept donations, and maybe Flattr will succeed where micropayments have failed.
There is also other transactions being made here, although it does not involve cash:
- Labor: Debugging and testing effort on platforms and in usage scenarios never envisioned by the original developers. By automatically tracking usage the developers get valuable information.
- Reputation: For many programmers, programming is ever so much about the positive feedback from making the software in the first place and people cherishing the result.
- Altruism: Making software products is relatively easy these days because of the availability of free and good developer tools and libraries. Releasing software back for free is one way of paying back to the community.