AAA Math
This web site provides interactive arithmetic exercises and problems. May be accessed by grade level or topic.

Education 4 Kids
Drill games -- the site is in transition. Small ($10.00 per "seat") fee for enhanced versions of drills

Math for Morons Like Us
A web site created by students for ThinkQuest, a project-based learning competition (http://thinkquest.org/). Very user friendly.

National Library of Virtual Manipulatives
This is a NSF supported project that began in 1999 to develop a library of uniquely interactive, web-based virtual manipulatives.

Visual Fractions
Created by a Middle School math teacher whose philosophy is "Rational Numbers are better understood when seen." This site uses graphics to illustrate operations on fractions.