Machine translation is an extremely useful tool for genealogists working with other languages, whether you are trying to navigate a website written in another language or read documents pertaining to your ancestors. The two most familiar tools are probably Babel Fish and Google Translate. The widest selection of languages is offered by Google Translate, currently 63 languages.
Google translate works by detecting patterns in documents that have already been translated by humans. The quality of the translation depends on the number of available documents. The best translations are going to occur for languages where Google has had access to a large number of translated documents, languages like French, German, Spanish and English. Machine translation is constantly improving, so if you haven't tried it recently you may be surprised by how good the results are.
Still, Google Translate is not perfect and you will occasionally get strange or nonsensical translations (such as an example given by James Fallows here). Genealogists need to pay special attention when place names or surnames get translated by the machine translator, because names are typically left untranslated. Google translate is a great tool for getting the basic gist of a document or for helping search for information. One useful feature of Google Translate is that it can be added to your Google Toolbar allowing quick and easy translation of web pages. However, when it comes to documents, you should always double check the translation the old-fashioned way, especially because the document might have structures or words on which the machine translator has been insufficiently trained.