The Online Geoboard runs from a set of Java applets. It's sort of a limited, poor-teacher's version of Geometer's Sketchpad. Sort of. But it's good. Among other things, for kids who do not have access to a geoboard or who have motor-control issues that make independent geoboard use difficult or impossible, it's very valuable. Check it out.

The National Center for Education Statistics is a site rich with mathematical information and activities for elementary and middle school students. Check out their dice-rolling program that will help you see that it takes LOTS of sampling before things approach their statistical probability.

The Virtual Library of Interactive Mathematics is a site based at the Utah State University that offers attractive, well-designed math activities for students from pre-K through 12th grade in an impressive assortment of topics. Measurement, fractions, algebra, probability, and more are at the student's fingertips (well, mouse buttons) as they move things around the screen and increase their understanding of the topics. (Thanks, Michele, for finding this one.) Although they say that a CD with the web content will be available for purchase, it is not yet, and I suspect that the project has ended without completion of that plan. The last update was in 2003. Still --  Highly recommended!

Powers of Ten is based on the ideas contained in a beautiful book of the same name. Move forward and back through orders of magnitude based on powers of ten -- galaxies, planets, continents, and on and on through smaller and smaller domains until you go into the negative powers and become too small to seel. Lovely!

Math Playground is a site aimed at students in grades preK through 6th, although some of our strongest students might find parts of it less than challenging. Still, there are lots of good activities here ranging from logic puzzles to math fact drills.

Mathematrix is a delighful site offering a wide assortment of mathematical fun, including flexagons, polyominoes, bubbles, art, and poetry. Great fun for students and adults.

MathCats is another "playful" site with ideas for crafts, LOGO projects (that use a dialect of LOGO different than the one we use in our classroom but which would still give some good ideas for interesting programs), and more.

The Geometry Center is no longer a real place, but this University of Minnesota site still contains the online archives, including free downloadable software (some of which runs only on mainframes, unfortunately) and a lot of things to keep you thinking about geometry. Definitely worth a visit.

Flexagon is a simple 2-page explanation of how to make a flexagon. It's a bit light on topological theory but very easy to use as a guide to creating and folding a flexagon through its faces.

Flexagons is a site that is not as clearly illustrated as we might wish but which does contain very well-explained information about making a flexagon and why it works. If you're exploring flexagons, it's a good place to visit after you have made at least one that works.

Paper Models of Polyhedra offers patterns for Platonic solids, Archimedean solids, and more. Requires Adobe Acrobat Reader (available for free download) for some of the patterns. Print from your computer on heavy paper or light card stock, get out your scissors, and have fun! Highly recommended!

Paperfolding.com is a site devoted to . . . folding paper. Mostly origami. It has a section called "Origami and Math" that may explain more than you ever wanted to know about the numbers that go with the shapes, but it's quite well-written. Lots of ideas for things to do.

The Math Lab is a commercial site with interesting materials for sale that would be useful at home as well as in the classroom. They also have a number of challenging activities posted on their site for free online exploration. You do have to register, however. Try their "dice of the gods" project, which shows you how to build the Platonic solids from old greeting cards using a compass and straightedge! There's a lot to do and learn at this site. Recommended.

Exploring Geometric Solids and Their Properties is a section of the NCTM "Illuminations" website. This is an interactive venture into geometric solids, showing them in both solid and wire-frame (transparent) form. Includes plans for hands-on work with simple materials. Great fun to play with online and good ideas for extended work with advice for teachers that could be equally useful to parents and independent learners.

Essays on Mathematical Topics is livelier than its title might suggest. Some of it is a bit of a stretch for our students, but it's written in a clear and friendly style.

Math Advantage is a site maintained by Harcourt, a publishing company. Great activities and a multi-media glossary, all sorted out by grade level. Fun as well as informative.

InterMath Dictionary is a really cool site for middle school students and their frantic parents. It gives clear, illustrated, often dynamic explanations of more than you ever wanted to know about mathematics. Fun to explore, even if your homework is done. Parts of this site are not finished, but that seems to be true of the universe in general.

The Math Forum is based at Drexel University in Philadelphia and is filled with useful resources for students, teachers, and parents. Things to do, things to learn, things to teach are all here. Highly recommended!

Natural Math One of several delightful web sites that present math as an adventure instead of a subject. A little hard to navigate but lots of fun to explore.

Math Mania This is a place that lets you explore some mathematical ideas from the view of researchers and experimenters. There are things for younger students as well as adults. Includes a glossary, biographies of mathematicians, problems to solve, and a tutorial.

The Knot Plot Site may not be "useful" in the most direct sense, but it does show that mathematics is a lot more than long division. All kinds of knots, including ones that will do the 3D trick if you look at them just the right way from just the right distance. Send them to your printer and you won't leave nose-prints all over your monitor screen.

History of Mathematics is a site that is still in the process of being put together. It's an ambitious undertaking, and the parts that are complete are done well.

Mathematics History from the Department of Mathematics and Statistics at the University of St. Andrews (Scotland) is another site not easily accessed by young learners independently but has wonderful information on mathematicians, the history of many mathematical topics, and the like. Another dangerous place in which I could spend an afternoon without ever getting to the thing I was trying to find out.

Channel1 is a collection of shareware and public domain material for many platforms. Not as large as the one that follows, but still worth checking out. Like any responsible download site, they are vigilant about virus-detection and other malicious code.

Ziff-Davis Publishing offers a wonderful site for shareware and freeware: safe, and efficient downloading, a good rating system for the programs, and an extensive catalog of games, utilities, educational products, program add-ons, and more. I use this site frequently and have rarely been disappointed with a program or its support. Especially recommended: WebFerret, an internet search program that works outside of your browser on your desktop. It's faster and more thorough than any other search utility that I have found -- and it's free! Just click on the linked name and you'll go right to the description page for it.

What-Is.com is a detailed glossary of information-technology terms. If you don't know your CMOS from your BIOS or someone has just called you as their lifeline from " ... Millionaire" to find out what a cancelbot is, this is the site that will educate you. Clear explanations, actually quite useful.