LanguageTool is an Open Office add-on that works in a very similar fashion to the MS Word spellchecker. However, LanguageTool not only corrects mistakes, it also provides an explanation as to why what the user wrote is incorrect.
Supported Language(s): More than 20 languages including English, French, Spanish, Polish, and DutchDownload Size (Mb): 86
System Requirements: Java 7.0 or later, OpenOffice
Price: Free
Website: http://www.languagetool.org/
Developer: Daniel Naber
Download: http://www.languagetool.org/