Many of these grants are offered several times a year, and/or are offered each year. If you have missed the deadline for one that looks interesting, put the location into "favorites" and check again for the next cycle.

Lowe's Outdoor Classroom Grants $2,000 - $20,000 Science education; gardens To provide outdoor, hands-on science education to students in grades K-12 and assist schools in enhancing their core curriculum in all subjects.
Advancing Student Achievement $50,000 Mentoring in math Provides mentoring in math for underachieving students. Lead mentor must be an actuary.
ING Unsung Heroes $2,000 (plus grand prizes of $5,000 - $25,000) Innovative classroom projects To implement or sustain "creative, unique educational programs...helping students to reach new heights."
NEA Foundation: Learning & Leadership Grants $2,000 - $5,000 Professional development Funds professional development for teachers (such as summer institutes, action research, collaboration with peers.
NEA Student Achievement Grants $5,000 Improve academic achievement; standards-based To improve academic achievement of students by engaging in critical thinking and problem-solving that deepen knowledge of standards-based subject matter.
State Farm/Youth Service America $500 - $1000 Service-learning Various grant opportunities, some for student-initiated projects, to design and carry out service-learning projects within schools.
Toyota Tapestry Grants $2,500 - $10,000 Science To fund innovative science projects within a school or district (K-12)
Save Our History Grant Program up to $10,000 Preserving local history and heritage To fund collaborations between schools and historic organizations (museums, libraries, historical societies, etc.) for preserving local history and heritage
PG&E Bright Ideas Grant $2,500 - $5,000 Solar science To develop and implement innovative solar energy projects in the classroom.