The best really was a perl web-based project. Cool in that it is perl and that it has plugins for many common repositories but the problem is how to collaborate. It is too asynchronous for me and doesn't allow a "story" which is the method that I have become accustomed to.
So I have started my own. Currently I have it hosted on GitHub:
The idea is to be able to display diff and be able to pin comments on the changes in the diff. A collaborator (or collaborators) can join your session and in real time see your comments and reply with their own creating threads of comments. I currently am thinking that there can be down to a 1 line granularity for comments. The core implementation is written in Scala and in the fullness of time intend to have a number of different User interfaces. At least one web-based and one Desktop based user interface. First version will only take patches but in time I want direct SCM integration