Emacs
An editor.
“Let them use emacs”: the interaction of simplicity and appropriation
An essay (PDF) by Michael B. Twidale and M. Cameron Jones on how Emacs is not readily appropriated and used beyond the way it was designed to be used or by people who were not expected to use it.
One reason for this might be that Emacs was designed to be extensible, so being outside of the range of ways it was designed to be used is a bit vague. The authors mostly criticize that the mechanisms of extensibility are complex, technically involved and out of touch with the needs of end users. They give examples of technology which is more readily appropriated for uses beyond the scope it was envisioned for.