GT.M. And since I'm at it, here's the third open source Mumps implementation. This one was once commercial and became free in 2000. So it has quite a few years under its belt and is not some hobby project by some enthusiasts, but is indeed commercially operated (e.g. for licenses on systems other than Linux and OpenVMS). If you read through the description, they've thrown the whole collection of buzzwords at the tool. Ok, for a language that simply maps its database as global variables into the programs, STM is so to speak already built-in. Even if that might be a bit of a cheat.
Archive 2. March 2013
Mumps. Yes, yet another implementation that seems to even implement some of the archaic job controls and other obscure artifacts. Well, since NoSQL, Mumps is probably respectable again. However, you have to compile this yourself, as there are no binaries directly available for OSX.
Mumps/II MultiDimensional and Hierarchical Toolkit. Yes, I'm back to absurd programming languages and found a Mumps that is open source and runs on various systems. I don't know what I'll do with it, but it will have to be something painful.