A freely available digital archive curated by Tufts University professor Gregory R. Crane. The Perseus Library features an excellent array of primary source material for the study of Ancient Greece and Rome.
Internet History Sourcebooks Project
Edited and curated by Fordham University's Paul Halsall, the Sourcebook is a valuable resource filled with "public domain and copy-permitted historical texts presented cleanly (without advertising or excessive layout) for educational use." The site is easy to search and has extremely useful 'Help' pages such as this one for folks looking to do research on the Ancient world.