The database WorldCat Dissertations and Theses lists many dissertations, theses and published material from around the world. Not as much full-text, but you can request items using Interlibrary Loan.
Search ProQuest Dissertations and Theses to identify many dissertations written at many universities. Not as exhaustive as Worldcat and does not include dissertations from Harvard or MIT. Does not include Masters theses. More full text available.