LanguageTool
LanguageTool is a free and open-source grammar checker, and all its features are available for download. LanguageTool website connects to a proprietary sister project LanguageTool Plus, which provides improved error detection for English and German, as well as easier revision of longer texts, following the open-core model.
LanguageTool was started by Daniel Naber for his diploma thesis in 2003.
It now supports 31 languages, each developed by volunteer maintainers, usually native speakers of said languages. Based on error detection patterns, rules are created and then tested for a given text.
The core app itself is free and open-source, and can be downloaded for offline use. Some languages use 'n-gram' data, which is massive and requires considerable processing power and I/O speed, for some extra detections. As such, LanguageTool is also offered as a web service that does the processing of 'n-grams' data on the server side. LanguageTool Plus also uses n-grams as part of its freemium business model.
LanguageTool web service can be used via a web interface in a web browser, or via a specialized client-side plug-ins for Microsoft Office, LibreOffice, Apache OpenOffice, Vim, Emacs, Firefox, Thunderbird, and Google Chrome. Its web app client can also be integrated on websites.
