Are there good, reliable reference resources on the Internet?  Yes.  This list is by no means complete.  It is intended to give you a feel for what’s out there.

American Society for Indexing 

Council of Science Editors

Fact Monster from Information Please 

General Tables of Units of Measurement (NIST)

How Many? A Dictionary of Units of Measurement

HyperPhysics, HyperMath, HyperBiology  (constants for science, math, and astronomy)

 The World Almanac e-Newsletter