Math.com
Computer Romanus: Roman Numeral Calculator
This site has a calculator that uses only Roman Numerals! There is also an explanation of the number symbols for reference.
Quia
Quia: Roman Numerals Strength in Numbers
Use this site to practice reading and putting Roman Numerals in order. You can get hints, check your answer, reorder answers, and get a new set of Roman Numerals.
Soft Schools
Soft Schools: Roman Numeral Conversions Worksheet
Generate a worksheet where students convert between Roman numerals and numbers. You choose the conversion type: Roman numeral to number or number to Roman numeral. The option is given to show the answers.