Google - Research database allows searching of the scholarly literature.
D. Bartlett - As titled, a concise guide to using the metric system.
Austin Community College - a resource of scientific notation problem sets.
National Institute of Standards - A guide to units and the metric system.
National Institute of Standards and Technology - Reference to metric conversion products and scientific sites.
Caroline Crouse Site - History and development of the metric system.
J. Proot - An interesting history of english units.
NIST - A history of U.S. efforts to go metric.
OnlineConversion.com - Over 5,000 units, and 30,000 conversions.
F. Senese, Frostburg State U. - From General Chemistry Online, tutorials on converting scientific units.
Ben-Cam Intermedia - An extensive unit conversion calculator.
A user friendly metric and other unit conversion calculator.
An alphabetical glossary of relevant scientific terms.
Digital Dutch - A fast, easy to use conversion calculator.
F. Senese - an interactive tutorial on measurement uncertainty.