General
http://illuminations.nctm.org/
Illuminations includes lesson plans, investigations and other resources provided by the National Council of Teachers of Mathematics (NCTM) that can be incorporated into classroom activities as well as links to the standards.
http://www.explorelearning.com
Exploremath - provides many applets that will examine - or provide information about - conics and other high school math topics when you choose interactive activities. Also provides some lesson plans.
http://www.mathforum.org/index.html
The Math Forum at Drexel University provide resources, materials, activities, and more to enhance math teaching and learning.
http://archives.math.utk.edu
K-12 math teaching ideas. Site contains links to many other sites that relate to math education.
http://www.pbs.org/teachers/math/
PBS site that provides resources for teaching K-12 math.
Fractions and Numeric Operations
http://www.visualfractions.com/index.htm
Work with fractions online
http://www.kokogiak.com/megapenny/default.asp
Megapennies deals with large numbers of pennies. This site helps visualize what huge quantities look like. It provides visuals and measurements to compare different quantities of pennies from 1 to 1 Quintillion Pennies.
http://www.enc.org/weblinks/
Government math lessons and activities from ENC Provides links to many and varied lessons and activities. Look at the dirty dozen. By the way, with some exploration you will find ideas and video clips that give you ideas for teaching.
Geometry
http://www.archimedes-lab.org/indexB.html
This site is for middle school and high school students, and allows them to see the visual creativity of geometry through puzzles, optical illusions, tessellations, tangrams, and more.