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.
Important announcement for users of KMail 1.0.x Are you still using KMail < 1.1.99? - it will stop working correctly in September 2001. Please read the official announcement.