OpenOffice.org 3.0 is going to be released next month and it features a grammar checker API. That is, a grammar checker can integrate itself into OpenOffice.org and check text while the user is typing text. The most recent release of LanguageTool, 0.9.3, makes use of this, thanks to the work of Marcin Miłkowski. It still has some bugs, but it is useful already:

screenshot of OpenOffice.org 3.0 beta with LanguageTool integration

In related news, the upcoming version of OpenOffice.org will not use the dictionary wizard (DicOOo) anymore, instead dictionaries for spell checking and thesauri will be installed as extensions. You can download those extensions from the official extension website.