An Open Source language checker for English, German, Polish, Dutch, and other languages. This is a rule-based language checker that will find errors for which a rule is defined in its XML configuration files. Rules for more complicated errors can be written in Java. You can think of LanguageTool as a tool to detect errors that a simple spell checker cannot detect, e.g. mixing up there/their, no/now etc. It can also detect some grammar mistakes. It does not include spell checking.
You can use LanguageTool as an OpenOffice.org plug-in or standalone.