Numbers
This site from the BBC has lots of activities for students to practice their number skills! They can practice their times tables, learn how to write big numbers, round numbers, learn about negative numbers, learn about place values and more with fact sheets, games, worksheets, and quizzes.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No Searchable: No

Numberland
Did you know that Hadrian's Wall is 73 miles long or that Germany borders 9 other countries? At this site from MathYear 2000 you can click on the numbers 1-100 and find fun facts about that number.
Intended Audience: Students Reading Level: Elementary School Teacher Section: No Searchable: No

Egyptian Mathematics
Learn more about the Ancient Egyptian number system. You can try your hand at solving some Egyptian math problems or do some math with an online Egyptian calculator!
Intended Audience: General Reading Level: Middle School Teacher Section: No Searchable: No

Fibonacci Numbers
This comprehensive site on Fibonacci numbers including a biography of Fibonacci; Fibonacci numbers and nature; The first 100 Fibonacci numbers and their factors; a formula for the Fibonacci numbers; puzzles involving the Fibonacci numbers; The Golden Section In art, architecture and music; The Lucas numbers; and additional links and references. 
Intended Audience: General Reading Level: High School Teacher Section: No Searchable: No

The Abacus: The Art of Calculating with Beads
The history of and how to use an abacus. 
Intended Audience: General Reading Level: Middle/High School Teacher Section: No Searchable: No

The Prime Page
This site contains: primers, articles, and information on primes; lists of prime numbers and prime factorizations; and software indexes and programs. 
Intended Audience: General Reading Level: High School Teacher Section: No Searchable: Yes

Roman Numerals 101
Are you stumped by the copyright years on movies? Confuses by which Super Bowl your are watching? Never fear! This site provides a primer on Roman numerals, Roman numeral games, and even Roman numeral flash cards!
Intended Audience: General Reading Level: Elementary/Middle School Teacher Section: No Searchable: No

Numbers in Over 5000 Languages
Learn how to count to ten in over 5000 languages. It's as easy as en, två, tre at this site!
Intended Audience: General Reading Level: Elementary School Teacher Section: No Searchable: No

What's Special About This Number?
What's the smallest perfect number? What is the only number that is twice the sum of its digits? Find out at this site with loads of information about the special qualities of numbers from 0-9999.
Intended Audience: General Reading Level: High School Teacher Section: No Searchable: No

The Secret Lives of Numbers
What are your favorite numbers up to? This site used public search engines and statistical techniques to determine the relative popularity of every integer between 0 and one million.
Intended Audience: General Reading Level: High School Teacher Section: No Searchable: No

Numbers From One to Thirty-one ...
Did you know that in England the number 11 sometimes appears in the decoration of Easter cakes? Find fun facts for the numbers 1-31 at this site! The site is from England, so there are many facts from English culture.
Intended Audience: General Reading Level: Middle School Teacher Section: No Searchable: No

The Digits Project
Fun facts about the numbers 1-9!
Intended Audience: General Reading Level: Middle School Teacher Section: No Searchable: No

Roman Numeral Converter
Enter a Roman numeral and this site will convert it or enter a number and this site will "Romanize" it!
Intended Audience: General Reading Level: Elementary School Teacher Section: No Searchable: No

The Largest Known Primes
Introduction to primes; the top ten record primes; a complete list of the largest known primes; other sources of prime information; notation and definitions; and Euclid's proof of the infinitude of primes. 
Intended Audience: Students Reading Level: High School Teacher Section: No Searchable: No

Can You Say Really Big Numbers?
Type in a number and this site will translate it into a word. Be brave! See if you can take it to the novemquinquagintillion level!  
Intended Audience: Students Reading Level: Elementary School Teacher Section: No Searchable: No

Cookie Dough
See if you can write numbers using words or numerals on checks at this game site from Fun Brain.
Intended Audience: Students Reading Level: Elementary/Middle School Teacher Section: No Searchable: No