VME
Surprised nobody has mentioned ICL's VME operating system for the 2900 and Series 39 mainframes. This was written from the ground up in S3, a high-level language designed by ICL based very closely on Algol 68.
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VME = Virtual Machine Environment, the native operating system for 2900 / Series 39. DME = Direct Machine Environment, this turns your 2900 into a 1900 (or System 4). CME = Concurrent Machine Environment, allows running both 2900 and 1900/System 4 environments at the same time. CME* is similar to CME but the DME environment runs inside a virtual machine on VME instead of being side by side.
The National Museum of Computing's ICL 2966, running George 3 and Maximop, also has Algol 60 and Algol 68-R compilers. The Noughts and Crosses and Game of Life demonstration programs are both written in Algol 60.
ICL's VME operating system is written in S3, an in house language designed specifically for systems programming, and very closely based on Algol 68.