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.

Kindergarten http://100thday.wetpaint.com/
2nd Grade http://ktoponce.wikispaces.com/
2nd Grade http://wikiharju.wikispaces.com/
2nd and 3rd Grade http://mswebster.wikispaces.com/
3rd Grade http://gowest.wikispaces.com/
3rd & 4th Grade Creative Writing http://terrythetennisball.wikispaces.com/
4th Grade http://mrssheftelsclass.wikispaces.com/
4th Grade http://4-4adventures.wikispaces.com/Worm+Online+Wiki
4-6 Grade http://arrrpirates.wikispaces.com/
5th Grade http://desbuffalo.wikispaces.com/
5th Grade Literature http://hobbitalk.wikispaces.com/

6th Grade http://mrlindsay.pbworks.com/
6th Grade World History http://mra-ancient-greece.wikispaces.com/
7th Grade Science http://ashscience7.wikispaces.com/
8th Grade Science http://team8bluesavestheworld.wikispaces.com/
8th Grade Language Arts http://stanleyyelnats.wikispaces.com/
Middle School http://discoveryisms.wikispaces.com/
Middle School http://collaborationnation.wikispaces.com/
Gifted http://talentedandgifted.wikispaces.com/