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. 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.