Math Open Reference is a site that explains the concepts of Plane Geometry, Coordinate Geometry, Solid Geometry, and Trigonometry. It has some great interactive tools for displaying a range of graph types and producing graph paper.
Sumdog is a math practice & game site that is free – more features available with free login; paid school subscription adds the ability to track student progress and identify student strengths and weaknesses. Covers addition and subtraction through fractions and percentages.
Carrotsticks has online multiplayer game to improve math skills; targeted at grades 1 to 5. Free for classroom use between 7am and 3pm and for addition practice any time. Targeting other math operations outside of school hours requires a monthly subscription.
Hoodamath is a free online math game site created by a middle school math teacher and his wife. It covers whole numbers, decimals, fractions, percents, algebra, geometry, and more. They have tutorials and movies to help learn particular concepts. They’ve also added word games, brain games, and general online games.
Johnnie’s math page has links to interactive math tools, math activities, and math fun. It covers number, geometry, fractions, multiplication, measurement, stastics and probability.
Colleen Young (also mentioned elsewhere on this site) has many links to math resources in her Diigo lists – including ones related to Bloom’s Taxonomy, Coordinated and Functions, GeoGebra, Puzzles & Challenges, and much more.
Tenmarks is a math practice program – free for teachers, has curriculums [sic] for grades 3 – High School mapped to state standards.
Some of the above are included in the blog post 11 Mathematics Resources to Try in 2011.