Wolfram Research
Wolfram Math World: Rational Number
This site gives an overview of rational numbers, gives interesting facts about the rational numbers including countability and probability of a rational numbers denominator being even. The site also contains related links.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Numbers
Math Forum response to request for information on various number sets. Also defined and discussed are integers, natural numbers, whole numbers, irrational numbers, transcendental numbers, and algebraic numbers.
Other
The Prime Glossary: Rational Numbers
This site gives a definition of what a rational number is. Gives examples and formulas. Very helpful.