LibreOffice 3.5 includes a grammar checker, called (or maybe based on) LightProof. From what I have read, LightProof seems to be a Python library that can be used to check for custom grammar rules. But I can not for the life of me find a project page for LightProof.
The closest I got was http://cgit.freedesktop.org/libreoffice/lightproof/tree/, which seems to be the code for the LibreOffice extension, not LightProof itself.
So is LightProof actually a library that can be implemented in other applications, or is it just a code word for a LibreOffice feature?
It's not possible yet, but: "we will be able to make a stand-alone version of Lightproof/Grammalecte as it won’t be necessary to use Hunspell anymore" - see Olivier R.'s post to the LibreOffice mailing list.