Please contribute your ideas and resources to this page.

Make your own graph paper.
Algebra Basics - Online tutorial on algebra, provides a step by step how to on basic algebra functions.
National Library of Virtual Manipulatives
NCTM Illuminations By: Marcopolo - 96 Online activities related to math
CoolMath - Online games lessons and calculators
Problems with a Point - Find problems you can use in your classroom.
Nick's Mathematical Puzzles - A collection of puzzles, hints, solutions, and links to related topics
NCTM Electronic Examples - Activities that Focus on the National Standards
Interactive Mathematics - Learn math by playing with it.
Nutshell Math - This is free for teachers to use. It is linked to MOST textbooks from 7th grade math through calculus. It uses Flash to do create a virtual whiteboard with a teacher giving lesson overviews and examples for homework. It does cost $$ for a school to use it for individual students.
Graphing Online Calculator
Hippocampus - resources (tutorials, simulations, guided practice) arranged by topic or following your textbook. Free registration and ability to create your own frontpage tailored to your course/students