Scholastic
Scholastic: Max's Math Adventures: Max's Measuring Mania
Read about how Max and Ruthie measured some fun objects in their gym. Use this method to measure things in your classroom. A helpful teacher's guide and extra challenges will make this a fun lesson on measuring with nonstandard units.
National Council of Teachers of Mathematics
The Math Forum: A Tour of Measurement
Peruse a collection of lesson plans, resources and FAQ's about measurement at every grade level.
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.