This is an excellent, concise white paper put out by the Information Services department of the City of Garden Grove, CA, regarding their wildly-successful use of Linux and other open source solutions for the city's computing needs.  As they mention in the white paper, the city saved $380,551.74 in initial costs by deciding to use a Linux-based server environment; they currently save $70,465.05 annually due to the use of open source software and solutions.

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As drawn from here.