"Facts are stubborn, but statistics are more pliable."
Dry/Liquid: Metric, U.S. Dry and U.S. Liquid conversions.
Length: All the usual suspects, plus nautical miles, angstroms, statute miles, furlongs, fathoms and more.
Area: Convert square miles, kilometers, centimeters, inches and more.
Percent change: This calculator from NewsEngin is too fast to be believed.
WebMath calculators: Calculators for percent, number problems, money, ratio, rate, age and mixture. Many more calculators on the home page of WebMath.com, including personal finance, units conversion, decimals to fractions and many more advanced calculators.
The Universal Currency Converter: Easy-to-use currency converter for about 90 global currency units including the euro. Beware: annoying pop-up ads.
The "Full" Universal Currency Converter: Expanded version of the aforementioned link. It contains information on 180 global currencies. It's fairly fast, but huge and thus unwieldy. Use the other one unless you need an obscure currency. Beware: annoying pop-up ads.
Heat index: If you know the air temperature and the humidity percentage, you can calculate a heat index.
Wind chill: You need air temperature and wind speed to use this nifty calculator.
Cricket chirp converter: Calculates the air temperature based on the number of cricket chirps heard in 15 seconds.
Conversion calculator links pages
WebMath: Math calculators figure everything from restaurant tips to physics problems. Fast.
Measure 4 Measure: Links page of conversion calculators from physics to cooking.
Micro Images math conversion tables: Links galore for all kinds of simple and advanced conversions.
Calculator links: Pacific Lutheran University Library's list of links.
Calculator links: Utah Link's list of links.
PMEL conversion calculator: A lot of everything in one place.
Conversion factors: If you're so inclined, you can check the math the old-fashioned way with these sophisticated conversion formulas. From PMEL.
Cautionary tale: This mathematically inclined guy issues a warning about trusting online calculators, at least too far to the right of the decimal point, anyway.
Statistics Every Writer Should Know: Explains mean, median, per capita vs. rates, margin of error and more. Easy to understand and useful, especially when interpreting studies and polls.
Statistical terms: Online glossary of terms.
Mathematical symbols: A brief chart explaining them all. (One disclaimer: On my browser, the "parallel" symbol appears to be incorrect.)
Encyclopedia of Mathematics: By Eric Weisstein. Scroll to the bottom of the page to search this comprehensive and very advanced site.
FAQ of sci.math: FAQ of the Usenet math newsgroup.
Math links pages
SumFun: From the serious to the humorous. Great links site.
Math in the Media
Glossary of Mathematical Mistakes: List of mistakes often made by advertisers, media, politicians, activists and others. Interesting.
How to Get Into the Newspapers: Kind of interesting article from Keith Devlin of FOCUS magazine of the Mathematical Association of America from March 1997. He's sharing with mathematicians how to successully submit story ideas (via press releases) to the press. In step 11, he talks about copy editors bungling the copy, but, what I find strange is, he goes on to say that if the article contains errors, complaints should not be made, rather, one should be happy to have achieved publication. How many people out there think this way?
Fun with math
Curious and Useful Math: Math candy for your head.
Math in the Movies: Math scenes in the movies. They have reserved space for the Unabomber movie. It's a hoot.
Math Jokes: Math-oriented humor with commentary. Humorous stuff, but just as fun to see what mathematicians find funny.
Data Powers of 10: From the byte to the yottabyte.
Numbers in Today's Date: Quick facts about the numbers 1-31.
Top: The Mona Lisa, 1506, by Leonardo da Vinci. I was going to link to the Louvre's Web site, but it's tres boring. Instead, check out this computer artist, Lillian Schwartz, who concludes that the model for the Mona Lisa was actually da Vinci.