A solitary developer at a tiny startup can use Drobox as a poor man's source code control system. In fact, that's what I started with to use as a just-in-case code backup. It's also super-convenient for syncing work between multiple computers (see my extended post on Dropbox). Not so great for team-based coding though.
Pretty soon though you'll want to incorporate a real source code control system--CVS, SVN, take your pick.
But either option requires a remote server to host your repository. You could run a Subversion server on your home computer, but then you've still got to worry about backing up your repository and having constant access (my Windows server at home crashes way too often and my home DSL goes down too often to be considered reliable).
And yet you're too small to start paying for a fancy source code control hosting service.
Couple Assembla's free svn repository with the free subclipse plugin to Eclipse and you get instant Subversion integration with your development code, synced against a Web-based, professionally managed repository.
My hope is that in the early going that subclipse will be a good enough Subversion client. I'm not anticipating the EssayTagger development team growing much beyond three people and each person will be playing in pretty different areas: Server-side code (me), UI elements (Mike), Flash ActionScript (contractor, probably).
I just integrated Eclipse and Assembla's svn repository this afternoon. As work progresses I'll post an update and a review of Assembla's service. Our needs are so simple though that we're hardly a rigorous test environment. Then again, that's how I like it: simple is good!
It's totally amazing how far a startup tech company can get on free tools and services these days.