Brainormous
More games - some free.

Clickmazes.com
Lots of Mazes!

Discovery Education - Brainboosters
Logical and lateral thinking as well as other “brain boosters.”

Discovery Games
Activities from the Discovery Channel - Cool!

Hotmath Games
Fun activities for tough topics.

Kid's Zone
Loads of activities

mathsisfun.com
Fun games. Student Recommended.

mathplayground.com
More fun games and links to even more fun!

Mazeworks
Mind bending and visual puzzlers to strengthen thinking skills.

National Library of Virtual Manipulatives or
Great interactive work to help students -- especially fractions;
Go to the Virtual Library and choose a subject and grade level.

PBSkids.org
Middle level games - FUN!

TheProblemSite
Interactive games for math concepts.

Puzzler's Paradise
Logic Puzzles and more.