Other scientific calculators
Google scientific calculator is good.
Wolfram Alpha is amazing. In fact, it is much more than a calculator.
Web2.0calc is also a nice scientific calculator.
Bluebit.gr is probably the best on-line calculator for matrix operations out there.
Digital Dutch has a very nice unit converter.
The Wolfram Alpha can also be used as a unit converter.
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 for your email) or from
another place (where they don't).