Math.com- Free math lessons and homework help.
Simple Sudoku- Great (free!) puzzle generator/solver.
About.com Math: Help and formulas, including basic algebra
National Library of Virtual Manipulatives: Online 'virtual' math manipulatives that offer hands-on visual math practice, Pre-K through Grade 12; Try 'em!.
MCAS Test Items (Massachusetts); Tests from 2007-2012, Grades 3-10, useful for practice, review, or just to see the types of questions included.
SAT "Game Plans" aimed at a target SAT score. Which questions to attempt, which to skip. Neat! From "The New Math SAT GAME PLAN" by Philip Keller.
MTEL Practice Tests (Massachusetts)
Superkids.com: Free Math Worksheet Creator
Wolfram Alpha- A Unique Computational Resource!