De Imperatoribus Romanis: An Online Encyclopedia of Roman Emperors
A List of the Roman Emperors (can be downloaded as a zip file)
The I, CLAVDIVS Project (an ambitious site that lists plot summaries for each episode, characters, Graves's historical sources, etc)
Another I, CLAVDIVS Plot Summary (less detailed than those at the previous site)
Suetonius' Lives of the Twelve Caesars: These biographies of Julius Caesar, Augustus, Tiberius, Caligula, and Claudius were major historical sources for Robert Graves in writing I, CLAVDIVS. Read them here online in English translation.
Which I, CLAVDIVS Character Are You?: The Online Quiz
The I, CLAVDIVS Drinking Game! (listed here for purposes of research only; of course this instructor does not encourage or condone drinking games)