Guidelines to creating magic-systems
hut.fi
This is about designing magic systems for role playing games and worlds. You can also find some discussion about magic in general.
City: A Medieval Spreadsheet
daisy.freeserve.co.uk
Population model for countries and cities, with explanations of the formulas, and a few samples, including Pern. Works, Excel, Quattro Pro, and Lotus 1-2-3 versions.