I've been using Dropbox for a while. It works like this: You sign up at their webpage for free to get 2gb space which you can access via their webinterface. If you want (and this is the novel part) you can install their software, which sets up a folder on your computer which is automatically (and in the background) synchronized with their server. You can share folders with other dropbox users. Underlying their system is a modified version of svn (I think), and you can go back to revisions via their webinterface. I don't know how concurrent editing is handled.

There are some features missing (selective synching, branching, milestones) but overall it's an awesome free service. The software is very well done and not distracting, I am synching folders from my office computer (linux) with my home computer (windows xp) and it works like a charm.