Government agencies
Grants.gov ...funding agencies include NSF, NIH, DOD, DOEnergy, HOHealth/Human Services, DOT, DOI, NASA, EPA
National Science Foundation - '...funds research and education in science and engineering, through grants, contracts, and cooperative agreements...' Also see their Guide to Programs.
Other organizations
GrantsNet- from AAAS '...find funds for training in the biomedical sciences and undergraduate science education...'
Sigma Xi - Grants in Aid - Funds travel and equipment for undergraduate and gradute research.
Foundations - a sampling of private foundations funding faculty or undergraduate scientific research
Camille and Henry Dreyfus Foundation - supports faculty in chemistry
Research Corporation - early career science faculty
Clare Boothe Luce Program - supports women in science, technology, engineering and mathematics
Howard Hughes Medical Institute Grants for Science Education
W.M. Keck Foundation
Arnold and Mabel Beckman Foundation
Bill and Melinda Gates Foundation
S.D. Bechtel, Jr. Foundation