MrDowling.com - 6th and 7th grade downloadable lessons
MacMillanMH Social Studies - Grade 6 (these are made to accompany their textbooks, but you might still be able to use them on their own along with other resources on the internet.
MrDonn.org -I used this site years ago with my oldest daughter. It has loads of kid friendly reading material, powerpoints and activities/projects. Suitable for multiple elementary grades.