The American Presidency Project is an invaluable source of information and sources for the study of the presidency. Election data, an extensive media archive, and online resources are provided.
The Miller Center of Public Affairs at the University of Virginia maintains a website devoted to the history of the presidency. Biographies of all the presidents and essays on the "president at work" are available. Materials acquired through an oral history project focusing on the key figures within all of the administrations since Jimmy Carter are accessible here, as well. A collection of presidential recordings dating to Franklin Roosevelt can also be downloaded along with their accompanying transcripts.
The "Famous Trials" website includes well-conceived pages dedicated to the impeachment trials of Presidents Andrew Johnson and Bill Clinton. A wealth of material is provided including trial records, transcripts of testimony, biographies of participants, and the impeachment articles.