I can’t really believe I have not written about ShareLaTeX before.
ShareLaTeX is an online editor of LaTeX files, with utilities such as previsualization, easy upload/download options, and what is even better, collaborative and free!
I’ve used it for years now successfully for my last papers, and I can say that it is easy, effective and works!
However, there are things I don’t like: there are some premium features not free, such as synchronization with Dropbox, track changes or connection with Github. These are amazing feats I would like to see in my free account. But I understand this is difficult to achieve unless Google comes and buy your site : )
In any case, I recommend it to everyone.