If you are into Latex, I was amazed at this site!

https://www.writelatex.com/about

The above reads like a dodgy spam message I know. But seriously if you're looking to use a text/code based document tool, Latex is pretty cool.

I'm just amazed at what people have managed to put together. A web based front end for Latex. Probably heaps more. I haven't started exploring it yet.

Cheers,

Chris