Pilot is cool because it's basically bare metal Mesa. Written in Mesa, runs only Mesa programs, and leverages features of the Mesa language for OS security (e.g. Mesa's capability system).
Other cool projects like this:
- Cedar was both a PL and an OS: https://www.ics.uci.edu/~andre/ics228s2006/swinehartzellwege...
- The Oberon OS and Oberon PL https://people.inf.ethz.ch/wirth/ProjectOberon1992.pdf
- A Scheme OS called MrEd: https://www2.ccs.neu.edu/racket/pubs/icfp99-ffkf.pdf
You might even throw the JVM and Smalltalk into this category.