Simon Fraser University
Chem1 Virtual Textbook: Understanding the Units of Scientific Measurement
As part of a larger site called "Getting started in Chemistry," this site examines the units of scientific measurement with specific attention given to SI base units, SI decimal prefixes, units outside SI, and more.
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.
Simon Fraser University
Chem1 Virtual Textbook: Error and Uncertainty in Measured Values
As part of a larger site called "Getting started in Chemistry," this site examines various topics related to measurements and the error and uncertainty that accompanies the methods of measurements.
National Institute of Standards and Technology (NIST)
National Institute of Standards and Tech: Si Prefixes
The official prefixes adopted in the US, as stated by the National Institute of Standards and Technology. Yoctograms? Zeptosecond? Yes indeed.
Other
Wfp Consulting: Si Navigator
A commercial site which has links to many SI/metrics sites. It gets you to a host of sites. Don't hesitate to use it for research.