I have been using Dropbox to share my files with reviewers of my book and it has been awesome. So good in fact, that I have moved all of my documents into it and use it for my regular repository. That way everything I add/edit is immediately uploaded there as a backup.
Twice now I have overwritten a file accidentally and was able to go onto Dropbox and recover the previous version. It is like having source control, if you know what this is from computer science, on all of my documents only it is seamless.
Really awesome tool/site. The only negative I know of is that it is pretty costly if you want to buy more space. Their lowest paid upgrade is 50GB for $100/year. I don't need that much space and it is way too expensive. I'll stick to the free account and get as much space as I can through referrals.
So, if you are going to use Dropbox, use this link and we both get extra space: http://db.tt/FfAXzVTH.