These are some wonderful educational wikis that have been reviewed by previous classes. Don't be constrained by the grade level. You may find a great idea that could be reworked for another age level.

2nd Grade
2nd Grade
2nd and 3rd Grade
3rd Grade
3rd & 4th Grade Creative Writing
4th Grade
4th Grade
4-6 Grade
5th Grade
5th Grade Literature

6th Grade
6th Grade World History
7th Grade Science
8th Grade Science
8th Grade Language Arts
Middle School
Middle School