The following were recommended by a colleague who teaches math.
Math.com: Recommended by a colleague who teaches math. Site has Math help at all levels: practice, tutoring, tools, games, and homework help.
SOS Math.com: "...free resource for math review material from Algebra to Differential Equations."
A-Plus Math.com: "...web site was developed to help students improve their math skills interactively." Headings include: Flashcards, Game Room, Homework Helper, Worksheets.
PurpleMath.com: Lots of help for students and resources for teachers. Lessons, homework guidelines, study skills, and links to reviewed internet sites.