vroma.org - A great resource for pictures and images of or relating to Roman history. It also has created a virtual Roman world, that allows students to more interactively learn about Rome itself, set in the year A.D. 150.
livius.org/rome.html - Contains links to short articles on a variety of topics and people of ancient Rome.
LacusCurtius - A fairly extensive set of links to pictures and images, Latin texts and their translations, subtopics such as military history, and more.
The Perseus Project - A gold mine for searchable Latin and Greek texts and their English translations. It also contains a full version of the Lewis and Short Latin dictionary and very helpful morphological tools.
De Imperatoribus Romanis - an on-line encyclopedia of Roman rulers and their families from Augustus (27 BC-AD 14) to Constantine XI Palaeologus (1449-1453).
The Roman Empire in the First Century - PBS website that provides a detailed look at important people, places and events from 1st century Rome.