Overleaf.com (previously WriteLaTeX) 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 (some unchecked since 2013):
ShareLaTeX is mostly open source. Easy to self-host on Sandstorm.
Overleaf doesn't require signup => Less friction when convincing collaborators to try it, just send them the URL and they can edit.
Overleaf's embedded preview uses images (fast but blurry), ShareLaTeX uses PDF.js. Both allow PDF download, of course.
ShareLaTeX has betterboth save history — shows, can show diffs.
- Overleaf free plan doesn't save full history, only when you explicitly create a version ?
- ShareLaTeX has a Track Changes mode letting you later reject/accept them.
both have some spell checking and auto-complete.
ShareLaTeX.com on paid plans 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.com on paid plans has [sync with Github](https://www.sharelatex.com/blog/2015/02/10/sharelatex-github-sync.html; Overleaf supports direct Git access.
ShareLaTeXOverleaf has "rich text" mode somewhat resembling LyX. Constructs such as sections, bullet lists, math are typeset inside the source.
Both have UI for adding/replying/closing comments. In Overleaf those are part of the source so the UI only applies in rich text mode.
both embedded chat, with math rendering.
both have forward/reverse search (SyncTeX)
Overleaf seems to work better on Android & iOS. UPDATE: unclear, Android typing in Overleaf is also problematic as of 2016. I think the only way I ever got it to work acceptably on Android was with external keyboard?
Overleaf has forward/reverse search (SyncTeX), ShareLaTeX doesn't.