Euclid's Elements

This site contains the entire contents of Euclid's Elements and with each proposition is a diagram you can manipulate in order to help you understand the proposition. Very cool.

What's my angle?

This is a nice little site to use for demonstrating how to use a protractor and for allowing students to get feedback on whether they are using the protractor correctly.

Quizlet

Of the flashcard sites I've explored, this one is the best I've found for allowing the input of mathematical notation. You need to register (free) to create cards, but others can use your cards without registering. There are lots of good options for working with cards you've created. The interface for basic card viewing and flipping works very smoothly and quickly. Want to try it? Experiment with my cards on sines and cosines and evaluating inverse trig expressions.

GeoGebra

I have only briefly previewed this free software created by Markus Hohenwarter for exploring algebra, geometry, precalculus, and calculus, but I'm excited about the possibilities I see there! The geometric construction tools are reminiscent of those in Geometer's Sketchpad. The algebraic tools seem more extensive than in GSP. Associated with the software is the GeoGebraWiki site, where educators can post resources related to GeoGebra.

EquationPlotter

Created at The Lawrenceville School, this is a great web-based tool for high school math teachers. It enables you to produce high quality, highly customizable graphs which can be integrated beautifully into word processing documents. In addition, the site includes many excellent demonstration applets of topics ranging from the unit circle to solids of revolution. Free registration is required to produce the graphs (but isn't needed to access the applets).

The National Library of Virtual Manipulatives

This site from Utah State University has a nice collection of accurately named virtual manipulatives that can be used in K-12 math education. These are organized both by mathematical topic and by potentially appropriate grade level. I would say that the target audience for most is probably grades 3-8, but there are some which can be used by younger or older students. Each manipulative has clear instructions and notes for the teacher or parent. Many also have associated activities which can be accessed at the click of a button.