MUMPS still has a healthy user base at that. MGH is obviously a large user base (interview with a hospital in Boston and it'll eventually come up in conversation), but the fun is when you get into the dialects of it. For example, MEDITECH has their own dialect named MAGIC, and at least one of their employees has attempted to make things better.
I should try to get him to post to talk about it, but he created an object-oriented version of MAGIC that supported longer identifier names and other more modern constructs, as well as an interpreter that spewed out perfectly standards-compliant MAGIC. Originally he was barred from using on existing, but eventually got approval to use it on new projects and I guess some portion of the MEDITECH developer population uses it over plain-jane MAGIC now since it's substantially easier to write and maintain.
Though recently he's reached out asking about the job market. He's been there since his early 20s and now is in his mid-30s and feels as if he's dead-ended himself on the job market unless he gains familiarity with something else. I think he's finally looking to break away from MUMPS.