Many textbook publishers post their books online and also include additional material and activities.  I have collected many of them here.  

Don't carry that book home if you can get it online!

Math
7th Grade Math
Algebra Readiness
Algebra 1

Science
Life Science - 7th Grade
Physical Science - 8th Grade

Social Studies
Medieval & Early Modern Times - 7th Grade
Creating America - Beginnings through World War I - 8th Grade
GATE Social Studies - Login Required

English
7th Grade Literature  Registration required.  See your teacher for access codes.
7th Grade Grammer:  Chapters 4-13  Chapters 14-24  Additional Resources
8th Grade Literature  Registration required.  See your teacher for access codes.
8th Grade Grammar:  Chapters 4-13  Chapters 14-24  Additional Resources