Simplest way to jointly write a manuscript? As with many papers in the sciences, there are multiple authors who aren't physically close to each other. I was curious about the most efficient and simplest way to technically share the writing. On the more complicated side would be LaTeX and subsequently using subversion to merge all of the changes. Then there is google docs. Finally, there is the traditional write out edits on word and email the document with comments back and forth.

What works?
If your collaborator is not technically inclined, then there are a few possibilities:


Google Docs
It is very simple, robust and real-time (no need of thinking 'which version is the most recent'). I created a lot of documents with it (though only 1 scientific article). A good idea is to use different colours so that it is clear who wrote what. One drawback is that it may be not as simple for equations and than you will need to copy things from there to LateX manually.
Annotating PDF files with Adobe Reader or sth equivalent
Also simple and robust. However, as it is not real-time you need to keep track of which version is current and it is not possible to make changes in the same time. Here you are the only person responsible for changes in LaTeX file.
Annotating or changing with OpenOffice, MS Word or another office package...
Another variant which allows you collaborator to make direct changes, at cost of some robustness.


As a practical remark for the two last: always keep track of the version, e.g. draft_20120424_prof.pdf, where you add both date of the last edit (not final5!) and name of the last editor. When exchanging things by e-mail it is easy to loose track which version is the most recent and in fact lost changes (often not knowing about that.)

Other possibilities, requiring some technical knowledge:


writeLaTeX or sth similar (ShareLaTeX, ScribTeX, LaTeX Lab - the one using GoogleDocs, ...)
Tools for editing  documents collaboratively (in real-time) and compile it within one's browser (so no need to install anything). Though some knowledge of LaTeX is required (at least to know how not to spoil a file).
ScribTeX(the only one I used) gives version control and you can see which lines were changed (as in version control system), so not need of guessing what might have been changed.
EDIT: ScribTeX is being replaced by ShareLaTeX.
writeLaTeX has live preview and may be the most suitable for beginners.
Git or another version-control system, e.g. at GitHub
However, it may require technical skills beyond edition of LaTeX. While it is not hard to use it, I never did I am using it for writing papers and I would never try to use it with someone with low computer literacy. 


When it comes to Dropbox - it is a good stuff for sharing files for a project but NOT for making changes in files. First, you never know when the changes happen. Second, you don't know where the changes happen.

There is also an article on it: Research tips - Online collaborative writing.

See also a few questions:


Simultaneous collaborative editing of a LaTeX file - tex.SE
Tools for collaborative paper-writing - MathOverflow
Compiling documents online - tex.SE