LanguageTool is a style and grammar checker that currently supports English, Polish, German, French, Dutch, and other languages to a different degree. It scans the words and their part-of-speech tags for occurrences of error patterns, which are defined in an XML file. More powerful error rules can be written in Java.
After the Deadline for OpenOffice.org is a smart English grammar, style, and contextual spelling checker for OpenOffice.org. The grammar checker highlights common errors in your writing. The style checker (off by default) finds passive voice, clichés, redundant phrases, and complex expressions. The contextual spelling checker looks for misused words and reports them to you. This extension communicates with an open source software service to check your writing. You may run your own server or use that provided by afterthedeadline.com. Text sent to afterthedeadline.com is sent over SSL and none of it is kept.