Mapping with friends - what tools available?

Hi all, me and my friend both like to play around with the hammer editor once in a while, and we were wondering if it’s possible to make maps together?

I’m thinking that real-time mapping together is out of the question (is there even any map editor that can do that?) So our best bet is to have a shared updated map-file, correct?
Does anyone of you know any tools that we would be able to use? I haven’t been able to find any info on it.

I use Google Drive to sync manifest files, but others swear by github or dropbox. That’s personal preference IMO.

Or just use a file server like megaupload or dropbox and you can either share the same map file or have it as a final version but start off with your own map and their own maps and create prefabs from it which are VMFs so they will load in just fine. I find it easy to do prefabs of buildings in one [prefab] map and make a layout of roads in the main map so it won’t overload with lag trying to make the buildings. ofcourse textures will probably be put in the necessary folders.