Almost all significant sites in the Classics can be accessed from one well-organized resource list, Maria Pantelia's Electronic Resources for Classicists: The Second Generation. Do visit, of course, the home page of the Washington University Classics Department.
Note: Greek linguistic and grammatical resources pertinent to Biblical Greek studies will be found on my Bible/NT/Early Christianity page; items more specifically pertinent to ancient history will be found on my Antiquity page.