Big Chalk - This is my favorite search engine for school related reference information.  They also have an excellent section for teachers that includes lesson plans and activities.

Marco Polo - This is linked to lesson plans and activities developed by leading educational foundations and associations (i.e. Kennedy Center for the Arts and National Geographic.)  Look for the Search MarcoPolo button on the upper right hand corner of the screen....happy hunting! 

Enchanted Learning - Geared mostly to the K-5 classes.  Can be a little slow to load but often well worth the wait.  

Education World - A good resource for everything from the latest grant information and educational research to teacher tested lesson plans and activities.

ThinkQuest - This site is composed of websites designed and written by students for students.  The variety in the quality of the sites, as well as the age appropriateness calls for careful preview by teachers.  It can be a great source for information written at the students' levels.

EduPlace - Houghton-Mifflin site.  Lesson plans, subject matter lists...great info.