SCIENTIFIC CALCULATOR
Tools
Other scientific calculators
Google scientific calculator is good.
Wolfram Alpha is amazing. In fact, it is much more than a calculator.
Unit converters
Digital Dutch has a very nice unit converter.
The Wolfram Alpha can also be used as a unit converter.
Reference materials
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 another place or from another place (where they don't).
Other links
If you are not 100% clear how scientific notation works and why we use it, this video explains it very nicely.
