One, a Google Docs app, LaTeX Lab (thanks to Daniel for pointing this out.) Another with https support Verbosus. I couldn’t get the later to compile and display in browser with FireFox, but did in Safari. Verbosus has an android app, but strangely no desktop version.
Recently I’ve been mostly using Dropbox for collaborations using LaTeX. Every once in a while there are conflicts in editing at the same time, but with only a few people this seems to work really well.