Robert Neuwirth writes (PDF) about the informal economy in Lagos, Nigeria, and its importance to the local economy. Local governments mostly view these informal markets in a negative way, even though they contribute significantly to the economy.
Some interesting points:
The demand for economic activities takes over everything as bridges are transformed into informal markets. The informal sector is the most dynamic and fastest growing segment of the economy. With very high unemployment, it is the informal sector that holds the country together. Cities need to be designed to accommodate street traders. This works a lot better that expecting traders to obey some sort of government rules.