Etherpad Lite is an updated version of Etherpad, an application for collaborative online editing. Each person writing shows up in a different color, there is a chat function, and it has a marvellous "history" function that replays how the document was produced. The document can be exported / downloaded in various formats.
We use Etherpad at the TH Wildau, where it is installed alongside Moodle for online learning.
But Pita Matischka has optimized the code to produce Etherpad Lite, which uses fewer resources and is available online for free. He presents it here:
What a great tool! I've introduced it to ELTABB, the teachers' association I now work for as events coordinator, to collaborate on reviewing events (see here).