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A very nice site utilizing all levels of precalculus and calculus is Temple University's Calculus On the Web online math series.

Please contribute your ideas and resources to this page.

Calculus Graphics - Graphics for use in the calculus classroom.
Fun Calculus Stuff - Tutorials and more!
Paul's Online Math Notes - Math Notes, Reviews, and cheat sheets
WinPlot - This a program that was recommended to me by a Calculus teacher. I am not very calc-savvy so I have no idea how to use it
National Library of Virtual Manipulatives
NCTM Illuminations By: Marcopolo - 96 Online activities related to math
Animated Math GIFs - from Wolfram MathWorld
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.
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