StructureMap was originally conceived in 2002 as the world's greatest object/relational mapping tool (and frankly a way for me to prove to prospective employers that I could actually code so I could move on from my non-coding architect position at that time). What exists today is the descendent of the overly grandiose configuration subsystem for that failed ORM concept (hence the name "StructureMap").
StructureMap was resurrected in the spring of 2004 as an "Inversion of Control" tool after I read about the http://picocontainer.codehaus.org IoC tool written in Java by several folks at my company at that time. In June of 2004 StructureMap went into its first production application and was also publicly released on the old Sourceforge.net website. As such, StructureMap is the oldest, continuously used IoC tool for .Net development.
The current release (3.*) represents the lessons learned from over 10 years of usage and other than a handful of class names bears very little internal resemblance to the original codebase.
StructureMap moved to GitHub in 2010 and is now distributed via Nuget.