Mecurial is an open-source version control application. It's free and written in Python.
Ranked in these QuestionsQuestion Ranking
Pro Easy to use
Mercurial is designed to make performing operations (such as commits, clones etc.) fast.
Doesn't require access to a central repository, because each checkout is a full repository by itself.
Pro Free and open source
Mercurial is open source and free to use.
Pro Good documentation
Pro Native Windows support
Native Windows support. No trace of cygwin or other dirty hacks typically found in competitors.
Pro Fully python based
Pro Same features Git provides, but much easier to use and to understand.
The command-line interface is more intuitive, requiring much less memorization and exotic options/piping to achieve the same thing you would in git.
Pro Doesn't delete history
Mercurial makes it pretty darn hard to delete history by mistake. In Git, if you mistakenly commit to HEAD and switch to a different branch, your commit is toast. Yes, you can dig through the reflog but most users don't even know of its existence and will wish they didn't once they find out.
Pro Actively contributed to by Facebook
Con Can't delete named branches
You pretty much have to use tags instead.