[WriteLaTeX.com](http://writelatex.com) is a service that lets you edit & compile  latex in the browser, with real-time edit merging.  
It's comparable to [ShareLaTeX.com](http://sharelatex.com/).  Notable differences (as of Oct 2013):

 - WriteLaTeX doesn't require signup => Less friction when convincing collaborators to try it, just send them the URL and they can edit.

 - WriteLaTeX's embedded preview uses images (fast but blurry), ShareLaTeX uses PDF.js.  Both allow PDF download, of course.

 - ShareLaTeX has better history — shows diffs.

 - ShareLaTeX has 2-way Dropbox sync allowing some offline/local editing (though only for one user; I tried it and wasn't impressed).  WriteLaTeX only has 1-way backup to dropbox.

 - ShareLaTeX has embedded chat.

 - WriteLaTeX seems to work better on Android & iOS.