Google Translator Toolkit is an powerful editor that enables translators to bring human touch to machine translation, aided by Google Translate. By using Toolkit's tools, you can translate documents stored on your computer, web pages, Wikipedia articles and Knol articles. After importing a document, Google generates the translation, displaying it next to the original text. You can select a sentence from the original document and Google lets you edit the translation. It includes collaborative features from Google Docs: you can invite other people to edit or view your translations, but the application doesn't display the collaborators that are currently editing a document. An interesting feature is the “translation memory”, a database of already-existing translations.
More info: Google Translator Toolkit