(Sample link) NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html
Math is Fun http://www.mathisfun.com/
AAA Math http://www.aaamath.com/
Math Playground http://www.mathplayground.com/
Holt Math Textbook http://my.hrw.com/index.jsp
Virtual Math Manipulatives Online http://nlvm.usu.edu/en/nav/vlibrary.html
Combining Like Terms http://www.math.com/school/subject2/practice/S2U2L4/S2U2L4Pract.html
Graphing Linear Equations http://www.math.com/school/subject2/practice/S2U4L3/S2U4L3Pract.html
Solving Systems of Equations http://regentsprep.org/Regents/math/solvin/LSolvIn.htm
Box and Whisker Plot http://ellerbruch.nmu.edu/cs255/jnord/boxplot.html
Box and Whisker Plots http://regentsprep.org/Regents/math/data/boxwhisk.htm
Stem and Leaf Graphs http://regentsprep.org/Regents/math/data/stemleaf.htm
Various Types of Graphs http://nces.ed.gov/nceskids/createagraph/
Practice with Fractions http://www.glencoe.com/sec/math/studytools/cgi-bin/msgQuiz.php4?isbn=1-57039-850-X&chapter=3&lesson=2
Biographies of Women Mathematicians http://www.agnesscott.edu/lriddle/women/women.htm
Middle School History Biography http://www.mathforum.org/library/drmath/sets/mid_history.html
Middle School Resources http://www.infohio.org/