Other
Salariya Book Company: You Wouldn't Want to Be a Roman Gladiator!
A web-book designed to show the pitfalls of being a Roman Gladiator. Although this is a cartoon book, it does give you some elementary knowledge about the Roman Empire.
Able Media
Classics Technology Center: The Roman Gladiator
Nice overview of the history of gladiatorial games and their function in Roman society. Mentions how people became gladiators, how they trained, and the possibility of female gladiators.