Fossil is the SCM I always wish I were using, but for the benefit of external collaborators, I stick with Git and GitLab (previously, GitHub). Fossil is a lovely, small piece of software that does exactly what I want in a nice and simple way. It's fast and uses SQLite for everything, making it easy to programmatically extend it and munge its internal data.
SQLite usage makes me wonder how will it behave over time with millions of commits. Also makes me wonder how will it handle distribution.
http://fossil-scm.org/index.html/event?name=be8f2f3447ef2ea3...
But that was, like, 2011, so I have no idea if it's still valid or not.