Grant Wrangler was the first resource that came to mind when I read the question.
Grant Wrangler is a free service that lists grant opportunities for teachers, school administrators, and students. Grant Wrangler also lists some contest opportunities. You can search Grant Wrangler by content area, grade level, and application deadline.
Applications for Education
If you're looking for funding sources for a classroom project or a school-wide initiative, take a look at what Grant Wrangler has listed.