If you want dirt-cheap (possibly free) backups for your source code, you can drop a Git or Mercurial repository in Dropbox and work from that. I’ve been doing both for 2 years or so and have never had an issue.
If you’ve never used either Git or Mercurial, I recommend starting with Mercurial and taking 20 minutes to read through Hg Init: a Mercurial Tutorial by Joel Spolsky.
Create your Git project as you usually would:
Create a bare repository in Dropbox:
Add the Dropbox location as a remote within your normal project:
Any time you want to push to Dropbox, run:
Create your Mercurial project as you usually would:
Create a Mercurial repository in Dropbox:
Edit the .hg/hgrc file within your repository, and add a couple lines that look like:
Whenever you want to push to Dropbox, run: