Latin Grammar, an on-line edition of Allen and Greenough's classic New Latin Grammar.
de imperatoribus Romanis, an on-line encyclopedia of Rome's emperors. This is a very good site for Roman Imperial history
Indiana Classical Conference, an association of high school and university Latin teachers
American Classical League, a group devoted to the promotion of Classical Studies
Society for the Promotion of Roman Studies, a scholarly association in the UK devoted to Roman Studies.