Skk.vim-B is a project mainly written in Vim Script, it's free.
Japanese SKK
This is a mirror of http://www.vim.org/scripts/script.php?script_id=3118
This script is an implementation of the Japanese SKK (Simple Kana to Kanji conversion program).
Get older versions from vimscript#1589. (Thanks to Noriaki Yagi for inventing this script!)
This plugin works under vim6, vim7.
Repository http://github.com/tyru/skk.vim/