Collaborative code writing tools
Visual Studio.Net has a plugin called wave-vs.net that allow real-time collaborative editing to support pair programming features.
Eclipse has two plugins called DocShare and Saros, that allow real-time collaborative editing of documents (DocShare) or projects (Saros).
(the above quoted from wikipedia)
Which processes benefit?
Obviously pair programming. Off the top of my head I can't think of any immediate show-stoppers when considering the improvement the GDocs experience brings, and extending that improvement to collaborative coding.
I think the collaborative approach would work best if multiple people are working on sections of code that can be considered 'close'. It can also improve round-trip time for changes, as you don't have to wait for a commit-update cycle to have the latest version.
Are they an alternative to VCS?
They are complementary. They both serve the same purpose of streamlining contributions from team members. But there is a fundamental difference in their approach: Current VCS'es mostly document snapshots of a project, while collaborative editing documents fine-grained changes, capturing the thought process of a developer.
Furthermore, commits in a VCS are neatly organised as a change set. In a collaborative environment, on the other hand, you can't even ensure the code will compile at any moment before completion of the whole project, because someone else might always be in the middle of writing a line of code. Change is continuous. Therefore it might not be possible to unravel the contributions into change sets.