National Science Foundation Graduate Research Fellowship (NSFGRF)

Supports up to three years of graduate study in the sciences and social sciences at a US institution. Eligible are US citizens, nationals, or permanent residents who are graduating seniors or recent alumni (without any graduate study to date) applying for eligible graduate programs, or those in the first or second year of such a program.
Deadlines vary by field of study, but are typically in late October-early November.