Other scientific calculators
Google scientific calculator
is amazing. In fact, it is much more
than a calculator.
is probably the best on-line calculator for matrix operations out there.
Unit and base converters
has a very nice unit converter.
The Wolfram Alpha can also be used as a unit converter.
is a very useful hexadecimal/decimal/octal/binary converter for programmers.
Fundamental Physical Constants
are provided online by the National Institute of Standards and Technology.
The "FUNDAMENTALS OF ENGINEERING SUPPLIED-REFERENCE HANDBOOK" is probably the best free engineering handbook on the web.
You can get it from the NCEES website
(where they ask you to register) or from
(where they don't).