RadixRoll is a neat pedagogic radix converter developed by ApplicVision. The application is available on both iPhone and iPad and lets you easily shift between decimal, hexadecimal, binary and roman numbers.
Hexadecimal: FFFFFFF (268435455)
Binary: 1111111111111111 (65535)
Roman: MMMCMXCIX (3999)
On iPad you can change all four radix scrolls at the same time, and on iPhone there's a slider to set which two number bases you want to display and convert between.