Other
Bipm: Si Brochure
Here's where to obtain your official brochure on SI, from the people who derived it. Little info at this site, except how to order an official brochure (and its cost). Or you can download a copy free.
CK-12 Foundation
Ck 12: Units of Measurement
[Free Registration/Login may be required to access all resource tools.] Students explore the scientific implications of measurement, and learn about system international metrics, dimensional analysis, scientific notation, and more.
Wikimedia
Wikipedia: International System of Units
Wikipedia offers detailed information on SI, or the International System of Units. Includes hyperlinked table of contents for easy site navigation.