My understanding and appreciation for Foucault comes from his idea of 'regimes of truth' in which he asserts that human understanding can a have reading of fact that is highly contingent upon social context.
As a specialist in information technology, I find the implications of this concept very useful in my role in knowledge management and business intelligence. I thus think of a kind of 'thermodynamics of consensus' which is something that requires energy to maintain, rather than something that becomes established over time in positive incremental quanta.
For example, the ideas that 'all men are created equal' or 'polio can be cured' or 'E=MC^2' are not plateaus of human knowledge per se. They are levels of understanding that must be pumped with energy to maintain. That's the energy of education and the energy to sustain an environment of education, and the energy to sustain the will to provide that environment. These become 'regimes of truth' and these regimes may rise to power or fall from grace.
Foucault in this way rather undermines wishful thinking of 'progress' because he understands that knowledge can be created or destroyed because it must be sustained. The genie may be out of the bottle, but it has to be inside a house or the wind will blow it away.
Foucault is misread, I think, by people who reside in very isolated bourgeois societies (houses) where the winds of discovery rarely blow. 'Regimes of truth' which maintain knowledge about the state of nature have no impact on nature itself. You cannot will yourself to levitate. Even if you spend billions of dollars to organize World Levitation Day.
Similarly Foucault has motivated millions to think about 'deconstruction of narratives' towards a greater understanding. You might deconstruct the narrative of an automobile commercial and see that in fact that the roar of Ford Mustang does not make you sexy, but that doesn't change the fact of its engine does roar. People have taken Foucault (heh) a bit too literally in thinking every narrative should be suspect and the only relaible discovery of truth involves deconstructing someone else's version of the truth.