I found this buried deep in an appendix of the Mapnik XML Schema Reference, and I thought it so useful I am reposting it here:
Zoom level Scale denominator
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**Update 2012-02-02 17:57 -0800:I just wrote a post about another way to do this.
I really like QGIS. It’s a powerful GIS that runs on Linux (among other operating systems) and doesn’t require incredible amounts of CPU.