A project (hosted by the wiki-translation site you are curently visiting) that aims at adding features to wiki engines in order to support that sort of thing.
Open source collaborative translation tools that can be embedded in any web publishing or web services platform to enable users to view, create and edit translations.
Translates interesting blogs in different languages. Translation is not done collaboratively at the moment, but the original content is generated in a somewhat collaborative way. (blogs).
"Open Translation Tools Convergence 2007, which will be held in Zagreb, Croatia from 29 November to 1 December 2007. Our goal is to map out Open Source Translation Tools and Open Content Translation Use Cases"