Darcs is a distributed vcs written in Haskell. It looks very interesting as it will probably fit very well with Harmony (even though Harmony uses OCaml).
Git is another approach to decentralized vcs.
DBinDBin builds on the "Minimum Self Contained Graph" (MSGs) (described in Signing Individual Fragments of an RDF Graph, WWW2005 poster (PDF)) which is implemented in RDFSync (part of the RDFContext Tools).
GVShttp://gvs.hpl.hp.com/application/about-gvs call RDF diffs "RDF Molecules".
They have an application demonstrating it: GVS Browser.
ArticlesManage source code using Git from developerWorks.
Hypertext Versioning (Versioned Hypermedia) in MolhadoA XLink-based scheme for a graph of revisions. http://home.eng.iastate.edu/~tien/molhado/vh.html
Relatedhttp://en.wikipedia.org/wiki/Unison_%28file_synchronizer%29 http://en.wikipedia.org/wiki/File_synchronization http://esw.w3.org/topic/HtmlDiff
The Source Code Management for OpenSolaris pages discusses their selection process and includes review/comparison of Git and Mercurial.