Converters & Calculators  

 Unit Converters 

MegaConverter lets you convert a myraid of things. I'm not going to place their device here, for it deserves its own spot in your bookmarks.

• Similar are The Foot Rule, Convertalot, Online Converter, eFunda & AskNumbers
  
• Convert-me also includes Russian, Japanese, Ancient Greek & Roman units
• Conversion tables at AllMeasures, SI Metric, A Dictionary of Units & Chemie.de
• A Dictionary of Units of Measurement

 Calculators 

• Simple Online Calculator
• Calculators On-Line Center - over 15,000 calculators!
• Matrix and Linear Systems - Matrix multiplication, matrix inversion & linear systems
• Calcoolate - calculation history & fast!

 World Wide Time 

The World Time Server - not only is it the best, it has free Atomic Clock software to download, to keep your PC time accurate

• Quick - The time in 30 major cities displayed in real-time, analog format
• Relative - Grabs your computer's time, and shows you the time everywhere else
• Visual - Select a timezone on the world map and see the current date & time. Uses Flash. Quicker way here
• Personal - This page simply tells you the time for place you customized, plus 4 USA timezones.
  With an easy to remember URL - WhatTimeIsIt.com

Also... Time and Date - very clean and useful...
the Official U.S Time, thanks to the American Gov't...
World Time Zones - ugly but useful...

 More Resources 

• Prefix Converter - like mega, milli, giga, pico, nano etc
• Engineers ToolBox - The EngineersToolbox is a freely-available collection of advanced online computational and reference modules for professional and student engineers
• Martindale's Reference Desk - 19,000 calculators!
• International System of Units
• Chinese Numbers <-> English Numbers - converts a number into Chinese or vice-versa, works for numbers up to 1 billion.
• Open Directory: Units of Measurement - more links...