The following offer "full-ride" fellowships that have up to 2-year durations. I've selected fellowships with themes that would be appropriate for my students.

American Association of University Women *International Students*
Environmental Protection Agency STAR Fellowships.
Fulbright Graduate Fellowships. * All Foreign Students*
NASA Earth System Science Fellowships.
National Science Foundation Graduate Research Fellowships.
Organization of American States. * Latin American Students*
World Bank Scholarship Program. * Latin American Students*

Long list of assorted science fellowships
Other nationally coveted fellowships