Darcs is a source code control system, but with a different feel from most. Where most systems (CVS, Subversion, Perforce, ClearCase, Arch, and so on) use client/server topologies, Darcs is purely peer-to-peer. With Darcs, every working tree is a repository, on an equal footing with every other repository. And connectivity between repositories is as easy and as varied as accessing files: you can use the file system, FTP, a web server, or even email, it doesn't matter. Changes flow from one repository to another via the "push", "pull", or "send" commands. There is no central authority.
I've recently started using Darcs for personal projects, and it fits that need admirably. I don't have to run a server, I don't have to checkout before I can make changes, and I can set up repositories on other computers as backups. I can even create a repository on a thumb drive!
I don't how well Darcs will scale up to large projects, or many developers, but for my personal use, it's perfect.