Comment 44 for bug 114375

Have you heard about Gramadoir?

It's an open source grammar checking engine developed by Kevin Scannel:
http://borel.slu.edu/gramadoir/index.html
I haven't tried it but it seems to be usable and well documented.

Kevin Scannel is also involved in the irish translation of LibreOffice.

There is an effort to integrate it on OpenOffice:
http://www.beta.scriobh.ie/Page.aspx?id=38&l=2

Hope that helps.