I used MUMPS (and MIIS) extensively while working in healthcare in the 80s and 90s. It was an efficient programming and database environment for mini-computers which combined a hierarchical database and interpreted language with sparse arrays and extensive pattern-matching capabilities baked in. It was widely used in hospitals for clinical operations and that legacy is still present in healthcare. The language itself would scare the shit out of anybody using modern technologies (self-modifying code, anyone?), but used with discipline it ran many applications that were literally a matter of life or death for patients. Some variants (MIIS, DEC) were also stand-alone operating systems running on the bare metal. It gave you lots of bang for the buck, even if the code itself looked like a printer test.
There used to be a small but active community of vendors and users. Today, there is one large player, Epic Systems, which dominates the applications market, especially in the electronic medical records area. They use Intersystems MUMPS (now known as M) as the underlying language; it has an extensive application building environment on top of the basic language to provide relational, Web and object-oriented abstractions. You can build applications in this environment without ever touching MUMPS code, though commercial applications will generally drop down into MUMPS for special purpose routines.