On Monday I had to prepare a class on collaboration in Digital Humanities, and while I was browsing here and there to find some example of collaborative DH project, I ended up in this very useful blog entry by Lisa Spiro. It is a blog entry, yes, but it is as good as an article.
The entry, you will see by yourself, not only lists loads of collaborative projects, but attempt a classification of them. I think this is the only example I have seen so far of a classification of DH collaborative project and I find it very well done. I thought it might have been useful for someone else beside me!