This site from Nova Roma provides great information on Roman Numerals. The text is complete, and a Roman Numeral Calculator is provided to do basic arithmetic. An explanation is provided for each numeral.

Explains Roman Numerals and offers hints about how to read them.

Explains the basic rules for writing numbers in Roman-numeral format.

Considers the origin of our "Arabic," system of number notation as probably having originated in India. Has illustrations demonstrating how much less cumbersome this system is than Roman, Egyptian, or Greek numeration.

Wikipedia offers detailed information on the Roman Colosseum, an amphitheater built in ancient Rome. Provides information on how it was built, what it was used for, and numerous images.