eFunda—Engineering Fundamentals
http://www.efunda.com/home.cfm
A huge, comprehensive site jam-packed with original engineering content and software. Thousands of pages with everything from engineering fundamentals to calculators, conversions, and all the formulas you need: fluid mechanics, mechanics of materials, plates, heat transfer, and many more.

Acronym Finder
http://www.acronymfinder.com/
Though this site is not engineering-specific, it can still help you decipher your way through those endless acronyms engineers love.

Online Conversion
http://www.onlineconversion.com
Thorough and comprehensive online conversion tools for dozens of quantities: temperature, speed, distance, volume, weight, and many more.

Environmental Engineering Glossary
http://civil.engr.siu.edu/ray/Glossary.htm

Pump Engineering Calculator
http://americanturbine.net/formulacalc/pump.htm
An online source to aid engineers in calculating necessary figures for water pump engineering. This site contains tons of calculators to help you wade through oceans of messy equations.

RPN Engineering Calculator ™
http://www.j-a-associates.com/rpn_calculator.htm
This great, easy-to-download calculator contains thousands of conversion options and dozens of helpful solutions. Just makes the job a little bit easier!  The download is free for 30 days, then registration is required.

A+ Materials Science and Engineering Glossary of Terms
http://www13.brinkster.com/justinmc/glossary/
A quick reference guide of engineering terms.

Process Associates of America Conversion Factors
http://www.processassociates.com/process/convert/cf_all.htm
A website full of automatic conversion tools for every measurement and calculation under the sun!