General

• AAAMath - hundreds of pages of basic math skills
• Allmath - flashcards, metric converter, games and lots of links
• Aunty Math - Fun math challenges for kids
• Ask Dr. Math - answers to questions about math
• Cool Math - games (including Lemonade Stand), puzzles, and special lessons on fractions
• Kid's Numbers - Math games, flashcards and more
• "Making Math Easy" series worksheets - Addition, subtraction, multiplication, division, fractions & decimals. Scroll down and click on "Free Worksheets."
• Math League Help Topics - help on dozens of math topics from 4-8th grade
• NRICH - monthly math problems, games and articles
• Quiz Hub - Math Facts, Math Magician, Math Problems, Math Tests
• Visual Fractions - instruction and practice in identifying, renaming, and operating on fractions.

Arithmetic

• Arithmetic Classroom Materials - links to materials for math teachers
• Dave's Addition Table - the classic grid for adding two numbers up to 10
• Dave's Multiplication Table - for multiplying two numbers up to 10
• Math Tricks to Learn Multiplication Facts - learn special tricks for learning multiplication tables!
• Memorizing the Multiplication Table 
• Quiz Hub - Math Facts, Math Magician, Math Problems, Math Tests. 
• Skill in Arithmetic - elementary addition, multiplication table, powers of 10. 
• World Wide Math Tutor - help with fractions.

Just For Fun

• The Abacus - the history and information about this amazing tool
• Brain Teasers - logic puzzles for grades 3-4 and 5-6
• CyberChase Games Central - lots of math related games from the PBS show
• Escape from Knab - develop financial savvy in a galaxy far, far away... 
• Figure This! - great brain puzzlers for families
• Lemonade Stand - learn to balance supply and demand selling lemonade
• Math Baseball - swing when you see the right answers! 

Codes - secret codes to make and break

• Codes and Ciphers - Information and practice on many different codes 
• Cryptology: the science of secret codes and ciphers - history and information on secret codes - history and practice exercises 
• Morse Code and the Phonetic Alphabets Dictionary - instructions in this alphabet signalling system
• International Marine Signal Flags - Maritime flags used as code 

Money

• Inflation Calculator - calculate past and present inflation rates
• Investing for Kids - stocks, mutual funds, and other ways to multiply your money
• KidsBank.Com - money and banking advice
• U.S. Treasury Kid's Page - all about bonds and other government savings programs
• Universal Currency Converter - calculates up-to-the-minute foreign exchange rates
• Young Investor - learn the basics and invest like a pro