I’ve found this through a mixi.jp community:
Ajax IME: Web-based Japanese Input Method
I’m simply surprised by its quality, especially considering its underlying libraries etc.
The idea was around for ages. Even myself written something like this before leaving to Canada back in 2002 (I was 17):
Romaji-Kana Converter on the Web
Well, about this my program, me and my friend (Sakuma, the guy running BitCoffee Inc.) have talked about the possibility of going further enabling it to handle Kanji just like the one above. We concluded it would be either Java or Flash client’s job to interact with the server. That was a few years ago.
The author of the above AjaxIME mentioned Sumibi’s web interface. This one works alright too. So cool! The technology behind those seem open-sourced NLP engines: probably ones from MeCab, Chasen, Juman etc. This is actually one of the subject areas I’m interested in.