Better SCM Initiative : Darcs
Note about Relevance
As of 2018, this site has become mostly irrelevant.
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Summary
Darcs is an open source (GNU GPL), distributed, version control system. It is easy to set up a repository for it, is based on an underlying "Theory of Patches", and it has some interesting features.
One possible drawback to it is that it is written in Haskell and has to be compiled with GHC. GHC is a Haskell compiler which is itself written in Haskell, with a problematic bootstrapping procedure, and Haskell may not be entirely scalable due to the limitations of the language. Another drawback is that it does not support marking files as executable.
Nevertheless Darcs is worth looking at.