Mercurial is a new version control system that specializes in distributed development. Its distinguishing feature is that there is no central repository -- all local repositories are peers (like Gnu arch). Synchronization occurs through change sets. It is growing in prominence because the Mozilla 2 team has chosen to use it in favour of CVS.
When reading up on Mercurial, I came across SVK, which is an extension of SVN to support distributed development.