LanguageTool is an Open Source proofreading software for English, French, German, Polish, Russian, and more than 20 other languages. It finds many errors that a simple spell checker cannot detect.
LanguageTool is freely available under the LGPL 2.1 or later.
The development overview describes how you can contribute error detection rules.
See 'easy fix' issues for issues to get started.
For more technical details, see our wiki.