Quoting the Monotone website:
monotone is a free distributed version control system. It provides a simple, single-file transactional version store, with fully disconnected operation and an efficient peer-to-peer synchronization protocol. It understands history-sensitive merging, lightweight branches, integrated code review and 3rd party testing. It uses cryptographic version naming and client-side RSA certificates. It has good internationalization support, runs on Linux, Solaris, Mac OS X, Windows, and other unixes, and is licensed under the GNU GPL.
Back in 2004, distributed version control systems were starting to gain traction (with things like GNU Arch being around). It wasn’t until I discovered Monotone that things finally “clicked” and I understood how such systems worked and why they would be the future. Git did not exist yet.
Monotone’s codebase was written in what-I-think-was pretty advanced C++ for its era. My primary contributions were the “big rewrite” of the built-in help system in 2007 and the maintenance of the packages for Monotone in pkgsrc, making it easy to set up a server (which I used to do at home).