GEOGEBRA: A great (and FREE) online graphing utility... use the online applet if you don't want to download it on your computer.
Khan Academy: A huge online educational resource... all free of charge!
Math Flash Cards: A-plus Math Practice Arithmetic and other basic math skills. There is also an option to practice operations with negative numbers.
Math Flash Cards: Practice your basic arithmetic skills online! (Addition, Subtraction, Multiplication, and Division are included.)
Math songs from other sites are listed in subpages: