Romkan.el is an Emacs package that converts Japanese Romaji to Kana and vice versa. It handles Katakana and Hiragana with the Hepburn and Kunrei-shiki romanization systems. Perfect for typing and learning Japanese!
This looks like a cute library, but to those who don't know yet: Emacs has a real Japanese input method: type C-x RET C-\ and then select `japanese`.