I haven't studied game theory, but it fascinates me. My intuition is that it isn't used by most "enterprise app" developers. However, it is clearly relevant to the large online sites (e.g. recommendation systems), and a huge influence on SO.
Have you applied any principles of game theory in your daily projects? If so, which principles?
I designed & wrote a review system and recommendation system for an online recipe-sharing website a long time ago. The recommendation system didn't require any game theory, just stats and sets, but thinking about the problems I had to solve with the reviews, they were definitely game-theory-y (though I didn't think about it that way at the time).
The problem that the site I worked on faced was that people could be both authors and reviewers, so there was a certain pressure to "pump up" their own recipes' stats perhaps by "pushing down" others. So there was a balance that had to grow organically from our rules to offset that selfish impulse; we did this with a per-user reputation score based on other peoples' reviews of their recipes and how helpful other people thought their reviews were. In retrospect I wish I had more rigorously applied some game theory here.
I am pretty sure that Hal Varian's work on auctions at Google must have a strong flavor of game theory, or at least micro-economics of decisions under uncertainty...