/. comments seem to be a contest to see who can demonstrate the least knowledge of the subject. While Mumps is largely unknown to many, it is widely used in health care and finance. Epic Systems controls about 40% of the US market (http://medicaleconomics.modernmedicine.com/medical-economics/content/tags/electronic-health-records/why-epics-market-dominance-could-stifle-ehr?page=full) and is written in Mumps as is the world's largest clinical information system, VISTA (http://worldvista.org/AboutVistA).
Mumps is a simple, string oriented scripting language for a builtin multi-dimensional and hierarchical database (http://www.cs.uni.edu/~okane/source/MUMPS-MDH/stonehill.pdf). It is especially well suited for applications such as medicine where knowledge is often as varying depth hierarchies that are not well suited for relational systems.
The commercial vendors are Intersystems (http://www.intersystems.com - their version is called Cache') and GT.M (http://www.fisglobal.com/products-technologyplatforms-gtm) whose version is open source / GPL. My own version (http://www.cs.uni.edu/~okane/) is also open source / GPL and can map the Mumps global arrays (the sparse array trees) to PostgreSQL.