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.
Hopelink
Hopelink: Teaching Percentages
Students can brush up on percentage skills with this lesson and calculate percentages using the "Percent Tricks method." For students to successfully use this method, they must have the ability to multiply and divide whole numbers.