Online supplement to the Guide to Grants link has not yet been updated
Available fellowships
In the first section below, we've listed nearly a hundred fellowships that we are familiar with, sorted by category; these are our suggestions as fellowships tutors on what to apply for based on your class year and your interests. In the second section are links to all the (extensive) resources we know about where you'll be able to find fellowships. Links to the calendar of deadlines are also included below. A 
denotes the resources we think are likely to be most useful to you.
Some common fellowships
Here are some of the fellowships that students commonly apply for. The fellowships in this list are highly desirable to many Harvard students and/or there are many of them available to Harvard students (and in most cases only to Harvard students). This list certainly doesn't cover all the fellowships out there—see the fellowship directories below for that—but it does cover nearly all of fellowships that we fellowships tutors are familiar with, and then some.
Fellowships listings
These listings contain most of the fellowships that you might consider applying for:
This is a brand-new feature from OCS that lists more than 150 fellowships (including nearly all of the ones above), which you can easily sort by who is eligible, what it funds, and the disciplinary area. (Notes, as of 06 December 2008: The site doesn't reset when you reload or revisit the page—make sure to click "Clear" at the top to reset it. Also, this site does not work properly in Internet Explorer 6; upgrade to Internet Explorer 7, or use Firefox, Chrome, Safari, etc.).
•Harvard College Guide to Grants, 12th edition (hard copy)
This book contains over 200 pages of fellowships listings. All the fellowships listed in the printed version of the Guide to Grantsare open to students at Harvard as well as other universities. The guide does not describe Harvard's application procedures, and it does not include any of the fellowships that are specifically for Harvard students. Several copies of the Guide to Grants are available in the Lowell House library and OCS reading room, or you can buy a copy at OCS for $5. Of the fellowships listed in the section below, nearly all of the the ones that aren't Harvard-specific have a helpful summary in the Guide to Grants
•Online supplement to the Guide to Grants
The online supplement to the Guide to Grants includes descriptions of all the fellowships available only to Harvard students (there are tons of these—you are very fortunate!) as well as some information about Harvard's application procedures.
•funding.fas.harvard.edu, the Harvard College database of Funding Sources for International Experience
This is a searchable database for study abroad, language study, internship, work experiences, research, senior theses, community service/volunteer work, teaching abroad, and purposeful travel.
•URAF concise list of external fellowships
This is a list of selected fellowships that administered beyond Harvard and open also to students from other universities. Many of these fellowships are listed in the drop-down tree above, but this list includes others as well.
There are many other fellowships out there. You may find some among the other references on the resources page of this website. There is also a long list of other resources in the back of the Guide to Grants (hard copy). There are other fellowships that are offered only to Harvard students but aren't listed in any of these guides; they're funded by departments or centers, or other schools at Harvard, and you'll have to contact them to see what if any fellowships they offer.
Deadlines
The majority of these fellowships are administered by the OCS Fellowship Office. For these competitions, the deadlines can be found on the URAF calendar.
For other competitions, to find out the deadlines you'll have to contact the fellowships administrators listed in the descriptions above.