The Toshiba America Foundation makes grants for projects in math and science designed by classroom teachers to improve instruction for students in grades K-12.
Maximum Award: $10,000.
Eligibility: Grades K-12.
Deadline: Decisions about grants under $5,000 (K-6: $1,000) are made on a rolling basis and applications are accepted throughout the year. Deadline for large grants grades 7-12: August 1, 2008.
http://www.toshiba.com/tafpub/jsp/home/default.jsp

The Siemens Competition in Math, Science & Technology recognizes remarkable talent early on, fostering individual growth for high school students who are willing to challenge themselves through science research. Through this competition, students have an opportunity to achieve national recognition for science research projects that they complete in high school.
Maximum Award: $100,000.
Eligibility: U.S. high school students.
Deadline: October 1, 2008.
http://www.siemens-foundation.org/en/competition.htm

The Mickelson ExxonMobil 2009 Teachers Academy offers a five-day program, with camps in New Jersey, Texas and Louisiana, designed to provide third- through fifth-grade teachers with the knowledge and skills necessary to motivate students to pursue careers in science and math.
Maximum Award: all expenses paid five-day program in July 2009. Eligibility: third- through fifth-grade teachers from all over the United States.
Deadline: October 31, 2008.
http://www.sendmyteacher.com/

American Institute of Aeronautics and Astronautics Foundation Classroom Grants are awarded to encourage excellence in educating students about math, science, technology, and engineering.
Eligibility: current AIAA Educator Associate or AIAA Professional members actively engaged as K-12 classroom educators.
Maximum Award: $200.
Deadline: N/A.
http://www.aiaa.org/content.cfm?pageid=244

RGK Foundation awards grants to programs that focus on formal K-12 education (particularly mathematics, science, and reading), teacher development, literacy, and higher education.
Maximum Award: $25,000.
Eligibility: 501(c)3 organizations.
Deadline for Letter of Inquiry: N/A.
http://www.rgkfoundation.org/guidelines.php#programs

The Actuarial Foundation - through its Advancing Student Achievement program - awards monetary grants to schools and groups throughout the United States and Canada. The basic requirement for schools or groups seeking funding is that they develop a viable mentoring program involving actuaries in the teaching of mathematics to children in private and public schools.
No deadlines
http://www.actuarialfoundation.org/grant/index.html

Dow Awards--by the Dow Chemical Company--awards grants to support proposed initiatives concentrating on math and science, teacher training, and parental involvement for K-12 school districts. School districts and school boards nationwide, as well as programs that promote systemic education reform in math and science, are eligible to apply. Special attention is given to school districts around communities where Dow is located.
Application Deadline: Varies
http://www.dow.com/about/corp/social/gen_fund.htm

The National Council of Teachers of Mathematics lists an array of Mathematics grants.  Check their web site for deadlines.
http://www.nctm.org/met.aspx?linkidentifier=id&itemid=198