Discovery Education Homework Help
(Note: the site opens on Math. Click the other subject tabs for additional Homework Helpers.)

KidInfo

Quiz Hub

Education World

HomeworkSpot

The Beehive
(click on the School link and look for K-12 Homework Help)

Infoplease: Homework Center

Math.com Homework Help
(click on the Select Subject links)

Homework Helper (refdesk.com)

Cramster

Shmoop

Chatterbee's Homework Help Center