LightWayText is a simple word processor. Easily as functional as MacWrite which was all I once ever needed. It was written by a Japanese programmer so it has features that can be hard to find in American programs. (It’s might not be as full featured as something as bloated as Word, but who uses all those features anyhow?)
It is available for both Mac and Windows. (You will need Japanese fonts and system support installed.)
It is shareware and costs US$25, (UK￡14). I don’t think the unregistered version is hampered too much but at such a low price for a program I’d encourage you to register and support the programmer’s work. (You could even surprise Yumi-san and make a nice comment in Japanese! You’ll get the most personable support you’ve ever gotten from a shareware developer.)
The web sites are a bit basic but the program is very good.
It is available as a Japanese version. (All menus and dialogs in Japanese.) For the very brave.
And is also available in English, French and Italian versions.