Sqlite filled a desperate need with no contenders, while Fossil, released in 2006, was going up against git and hg (both released a year earlier). Though Fossil is much more than a SCM (wiki, bug tracker, blog) and while not mainstream, is still used by tcl.tk and others.
SQLite itself is hosted on fossil, as is Tcl/Tk as you mention (and it's various satellite projects), and I know NetBSD was flirting with it too (no formal decision made yet on migrating from CVS afaik). I use fossil for all of my personal projects. It's more than capable, and a joy to use. Everybody should try it for a week. http://www.fossil-scm.org/
