From the webpage:
Groonga is an open-source fulltext search engine and column store. It lets you write high-performance applications that requires fulltext search.
The latest release is 1.2.5, released 2011-08-29.
Most of the documentation is in Japanese so I can’t comment on it.
Think of this as an opportunity to (hopefully) learn some Japanese. Given the rate of computer science research in Japan it will not be wasted effort.
PS: If you already read Japanese, feel free to contribute some comments on Groonga.