gc3
Gram's Commander 3
Gram's Commander was a popular two-pane file manager for MS-DOS and UNIX that I wrote around late 1980's-1990. It was distributed on a number of Usenet sites and had quite a lot of users, but after GNU Midnight Commander was created I decided to retire it. Unlike most such applications, it embeds a custom DSL and many of the commands are implemented in a script written in that DSL (see gc3.key).
I started work on a v4 that would have replaced the custom DSL with Tcl but never completed that. I keep swearing I'll one day do it with Python.