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.