Below, you'll find a fairly comprehensive list of the major scholarships available to undergraduates, graduating seniors, and graduate students. These are divided by category and linked to their official websites.

Arts and Humanities

Jacob Javits Fellowship (arts and humanities)
Online application open – August. Application deadline – early October

British & Irish Graduate Scholarships (begin working with scholarships office in the spring)

Churchill Scholarship (Cambridge University)
Gates Cambridge Scholarship (Cambridge University)
Marshall Scholarship (United Kingdom)
Mitchell Scholarship (Ireland and Northern Ireland)
National Institutes of Health - Oxford & Cambridge
Rhodes Scholarship (Oxford University)
Clarendon Fund (Oxford University) 
Saltire Scholarship (Scotland)

Environmental & Native American-Focused

Morris K. Udall Scholarship (second- and third-year undergraduates)
Campus deadline – mid-January. External deadline – early March
NSF - Clean Energy Grants
Sustainable Agriculture Scholarship (Annie's)
Doris Duke Conservation Fellows Program (master’s students committed to careers in conservation)
Brower Youth Awards (Earth Island Institute)
Eckenfelder Scholarship
Garden Club of America Scholarship
National Environmental Health Association
Middlebury Fellowship in Environmental Journalism
Wells Fargo Scholars (American Indian and Alaska Native students pursuing an education in business)

European Union

Erasmus Mundus (fully funded master’s programs that place you in several EU countries over two years)

International Students

Opportunities for non-U.S. citizens

LGBT

Point Foundation Scholarships
Joan Heller-Diane Bernard Fellowship in Lesbian and Gay Studies

Military

Pat Tillman Foundation Leadership through Action

New Americans

Merage Foundation American Dream Fellowship (funding suspended)
Soros Fellowship for New Americans
External deadline – early November

Public Service, Policy & Leadership

Carnegie Endowment Junior Fellows Research Assistantships
Campus deadline – mid-December. External deadline – mid-January
Pickering Foreign Affairs Fellowship - Undergraduate and Graduate
Online application deadline – early February
Public Policy & International Affairs Fellowship Program
Scoville Peace Fellowship in Washington
Application for spring 2008 fellowship – mid-October. Application for fall 2009 fellowship – early February
Harry S. Truman Foundation Scholarship (third-year undergraduates)
Campus deadline – early November. Campus interviews – mid-November. External deadline – early February

Scientific Research, Mathematics & Medical School

Barry Goldwater Scholarship (undergraduate)
Goldwater campus deadline – mid-November. Goldwater external deadline – first of February
Department of Energy Science Office Graduate
Hertz Foundation Scholarship (hard sciences)
External deadline – late October
J. Edgar Hoover Scientific Scholarship (scientific research applicable to law enforcement)
Campus deadline – late-January. External deadline – April 1
Howard Hughes Medical Institute Predoctoral Fellowships
National Health Service Corp Medical Scholarship
National Institutes of Health – Graduate Partnership Program
National Institutes of Health – MD/PhD
National Institutes of Health - Oxford and Cambridge
National Institutes of Health - Undergraduate Research Scholarship Program
National Institutes of Health Post-Baccalaureate IRTA Program
National Science Foundation Graduate Research Fellowship
(Enter “Graduate Research Fellowship” in the search field.) External deadline – early November
Tylenol Scholarship - medical-related career focus (undergrads)

Travel-Study

American-Scandinavian Foundation Fellowship
Online application – deadline November 1 (decision mid-March)
American University in Cairo Presidential Internship
Deadline end of April (notice mid-May)
Chinese Scholarship Council (post-graduate, 2-7 years of funding)
DAAD Scholarships in Germany (undergrad and grad)
Numerous scholarships, deadlines vary
Fulbright Scholarship
Draft essays by September 1st for feedback. Campus deadline – late September. Campus interviews – early October. External deadline – mid-October
German Chancellor Fellowships
External Deadline – October 31
Gilman International Scholarship Program (undergraduate)
External deadline for spring cycle – early October. External deadline for fall cycle – early April
Boren Scholarship/Fellowship (National Security Education Program)
Campus deadline – early January. Campus interviews – mid-January. External deadline – early February
Rotary Ambassadorial Scholarship
Rotary Ambassadorial Pre-Application – early April

Other Graduate Scholarships

James Madison Graduate Fellowships (teaching U.S. Constitution and history)
External deadline – first of March
Phi Kappa Phi Graduate Fellowship
External deadline – February 1