These sites are maintained by other offices or universities and for this reason they duplicate each other to some extent. It’s worth browsing through at least one of them to see if there is something that suits you.
Columbia Fellowship Web Site which provides information on applying for fellowships as well as a searchable database of fellowship opportunities.
Cornell Fellowship Database provides by far the longest list of opportunities, including lists of summer and postdoctoral opportunities; searchable as a database, contains detailed descriptions of programs, as well as links to their web sites.
Duke University lists fellowship opportunities for graduate students, postdoctoral fellows, and faculty in the arts, humanities and social sciences.
The Linguistic Society of America (LSA) provides a grants calendar and list of grant agency addresses to LSA members from the “Guides to Grants, Fellowships and Awards” link on the LSA resources page. (This list includes graduate opportunities, but is not limited to them.)
Stanford University’s List of Fellowships (A web site designed for advanced undergraduates, which includes links to the major fellowship programs; most of these opportunities are subsumed under other fellowship listing sites.)