Math
All Levels .
Shodor Interactivate. Sophisticated interactive games and activities (K-12): Number and Operations | Geometry | Algebra | Probability | Statistics | Modeling. Great graphics plus: Tools | Dictionaries. (Asks for a tax deductible donation.) http://www.shodor.org/interactivate/
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Illuminations is the National Council of Teachers of Mathematics‘ website. Pre-K through 12th games and activities, arranged by grade level. Use terms to narrow search. Icons help you choose. Just play, or click Exploration for info. http://illuminations.nctm.org/ActivitySearch.aspx
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Virtual Manipulatives. Fun, interactive geometry manupulatives for K-12. Mazes | Tangrams | Geoboards | Tesselations | Transformations | Programmable Turtle 
| lots more. (recommended by NCTM & an NSF award winner. ) http://nlvm.usu.edu/en/nav/topic_t_3.html
Pre-K .
Math for Kids. Activities, Games and Practice for: Counting | Time | Simple Math Operations. Needs an adult to navigate. http://www.pitara.com/activities/math/
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KidsNumbers. Organized by lessons. Arranged in a logical order. (or just click something that looks like fun) Great audio & graphics. Scroll past the introduction for easy clicking. http://www.kidsnumbers.com/counting_1_4.php
K - 4ish .
Click on bricks. Fun multiplication practice. http://kathyschrock.net/clickonbricks/
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Edu4Kids. Loaded with Drills, Practice, Games for: Basic Arithmetic | Time | Money. Adult needs to set it up. Minimal graphics. Thorough and varied content. http://www.edu4kids.com/index.php?page=12
Middle School
Prentice Hall website. Goes with your text book. Great Homework Video Tutor | Online Practice | connections to your textbook. (you need the webcode from your book for some of these.) http://phschool.com/atschool/phmath07/program_page_ms.html
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Check out the sites in All Levels (above). There are just not a lot of sites only for middle school. (It’s so unfair!)
Algebra .
GoHRW textbook site (Alg I) Go to Homework Help –> your chapter –> lesson –> Tutorial. It’s GREAT HELP. Then practice! You’ll be glad you did. There’s also Bilingual Glossary | Chapter Projects | Lab Resources| Refresher Course. http://www.gohrw.com/gopages/ma/alg1_07.html
(and Alg II): has the same stuff at http://go.hrw.com/gopages/ma/alg2_07.html
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AlgebraHelp. By Topic: Explanations | Examples | Worksheets (you can even print one for trinomials). Coolest feature: A calculator that works a problem for you. http://www.algebrahelp.com/
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MathNets. Scroll to purple box –> Find your topic (next to the purple box) –> Click your grade level number –> excellent interactive practice. http://www.mathsnet.net/algebra/index.html
Geometry .
ClassZone is the site for your SISD textbook. It includes Animated Geometry | HomeTutor | Practice, Practice, Practice | Games | Practice Quizzes | Reference stuff like formulas. Give it a shot!
http://www.classzone.com/cz/books/geometry_2007_na/book_home.htm?state=TX
MathNets is interactive geometry for activites like: Construction | Transformations | Shape. Or by topics like: Euclid’s Elements | Sacred Geometry | The Hidden World |Egg Project |. Cool to try, or look lower on the pages for practice with various concepts. Really interactive! http://www.mathsnet.net/geometry.html
Everything more complicated
Ask Dr. Math. Intense. For people who know the basics and are looking for answers to tough questions. Very thorough! http://mathforum.org/dr.math/dr-math.html
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Highlights for High School. This is Massachusetts Institute of Technology! Open for you. Including Build Stuff | Save the World | Write Better | Labs | Competitions | Demonstrations on Video. http://ocw.mit.edu/OcwWeb/hs/home/home/index.htm
Or if you’re really serious visit the main OpenCourseWare site at http://ocw.mit.edu/OcwWeb/web/home/home/index.htm
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You math experts will have to let me know about other sites. I’m afraid to look.
