Collaboratively writing e-mail (sharing mail drafts)
A test: I gave one GMail account edit access to another GMail account (yes, you can do this) and started editing an e-mail, saved it as draft, and then went to the other account— the draft was there, but not only was there no autorefresh of the other's changes when i edited simultaneously, but it forked and created two drafts of the same e-mail.
Etherpad seems to be in the Debian repositories which makes it very attractive. Plus they are actively trying to get it to integrate with Wordpress so we should jump in there too.
There is a stab at an Etherpad integration Drupal module. There is also an intermittently active thread on Drupal and Etherpad in the LA Drupal group, and an even longer thread on the interest in a module to provide Drupal Etherpad Integration in the contributed module idea and education-related groups, with some thoughts on how to do it simply, such as with an iFrame.
The previously-mentioned module's active code lives on Github, and the author says it is basically working:
That author is no longer interested in the module, however, and Mike Gifford of OpenConcept wants to drive it forward in Drupal 7 but does not have time to take it on himself either. We should be able to work together to Snowball this into a real and active project.
Regardless, having any Drupal/Etherpad integration would seem to require running Etherpad itself somewhere, so let's start with that:
Collabedit looks like it works just fine online and it does have a download button.
Stixy looks irredeemably commercial.
SynchroEdit seems to have no activity since 2007.
WriteBoard from 37 Signals is free, but not Free and it does not have an API advertised.
http://gobby.0x539.de/trac/wiki/Download (only precompiled for Windows and Linux, not Mac)
http://www.codingmonkeys.de/subethaedit/ (but Mac only)
DimDim was acquired by Salesforce. RIP.