LanguageTool is a rule-based language checker that will find errors for which a rule is defined in its XML configuration files or Java code.
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
It has been integrated in OpenOffice's latest 3.0 release, but can be used under 2.x also.
Install instructions in OpenOffice 3.x
Double click LanguageTool-0.9.4.oxt to install it. If that doesn't work, call Tools -> Extension Manager -> Add... to install it. Close OpenOffice.org and re-start it.