Without a doubt, Heidegger's explanation of readiness-to-hand in Being and Time has done more for my usability and process inefficiency evaluating skills than even Don Norman's Design of Everyday Things or any HCI book I've ever read. While Norman's book comes in a strong second, the benefit of the first few chapters of Being and Time is the incredibly low-level cognitive access it gives you to the work your brain does when engaged in a task. (If you're interested and don't mind slogging through some tough writing, pick up the hardcover version and Dreyfus' Being-in-the-World and read them together. Reading a Wikipedia article or some other summary fails to deliver the same degree of low-level access.)

I'll also mention Ayn Rand as someone who wrote inspiringly about the entrepreneur as a (potentially) heroic figure, but hasten to add that her sociology of looters and movers is way too simplistic to ground some of her views on ethics.

John Stuart Mill's "Utilitarianism" was my first assigned reading in a political philosophy class, and remains my favorite.

Nietzsche's On the Genealogy of Morals prompted me to question whether the values I had always held were really all that valuable.

Eleanor Rosch's work on basic categories and prototype theory (without which Lakoff's Women, Fire, and Dangerous Things--not to mention Weinberger's Everything is Miscellaneous--would likely not have been written) is fantastic, debunking empirically the Aristotelian theory of categories as being defined by necessary and sufficient conditions (they can be, where we intentionally define them to be from a top-down point of view, as in geometry, but bottom-up the construction of categories is much looser and based on family resemblances with one or several prototypes serving as the central point of comparison: hence, when I say "bird" you're more likely to think of a bluebird-type bird than a penguin or an ostrich). Knowing this does a lot to help loosen up your understanding of things, which can become rigidified by traditional mathematical, scientific, and analytic philosophical education.

Several others worth mentioning, but I've got to get back to work!