Extol Ohio.com
This site is full of metric information. Data tables, charts and information. EXCELLENT!

Friction loss table for various fittings from pump.net
Friction loss tables in imperial units.

Gas laws textbook.
This site contains extensive educational information on thermodynamics laws. NICE!

Heat transfer theory.
Excellent resource which covers heat transfer theory, devided in sections.Conduction, convection and radiation. EXCELLENT!

Maryland metrics
This is a commersial site, but includes huge amount of engineering information. Data tables on fasteners, flanges, gear design, material properties, calculators and much much more. EXCELLENT!

Monachos engineering calculators.
A collection of powerful online engineering calculators and resources. Unit conversion , voltage drop , pipe friction loss , data tables and more...

Natural gas information portal.
All you need to know about natural gas. Links to other internet resources included. EXCELLENT!

The Fridge.com - Accoustics information
Excellent webbook on accoustics. Contains lots of information both for the novice and the expert. EXCELLENT!

The Fridge.com - Heat transfer information.
Excellent information on heat transfer. Covers many sides of the aspect, and is very useful for the novice and the expert. Full of informative pictures and diagrams, excellent work. EXCELLENT!