The most popular search terms that drive traffic to this site are in the title. I bring it up again because recently my students were realizing the need for source control. While they are learning to be web developers and experience with a Version Control System will be essential down the road, as they are only beginners right now, it's an added complication they don't need.
So I explained to them how for my private projects I use Dropbox as a poor-man's CVS, they were very excited by the prospect of being able to access their code both in class and from home in addition to gaining free version backups.
There is a lot of criticism of this kind of idea in the Dropbox forums and there is a lot of truth to it. This is no replacement for a real Source Control system for a team of developers or even a lone developer who will suffer financial loss if his codebase rots. This is only a convenience for someone who otherwise wouldn't be using a Source Control system.
This is especially important to a budding developer who makes one accidental change and then goes through hours of progressively destroying their one file full of code trying to undo that mistake. For this particular use case, Dropbox is a perfect solution as a CVS.
Otherwise you might want to check out git and github as a great solution to storing your code centrally and tracking any changes you've been making. Unfortunately if you'd like to keep your code to yourself you will need to pay for the privilege 7$/month is a lot though, to me anyways.
The other reason many people search for CVS and Dropbox is in order to learn how to store your repository files (of an actual Source Control system) in a Dropbox for backup purposes. This thread on Dropbox's forums has a discussion of this. Be warned according to someone in the forum it has been known to fail fantastically, all of a sudden.