AdaOS - Goals: create new OS mostly written in Ada language, to help promote Ada; use typical modern, pragmatic methods to design it; based on a microkernel supporting efficient IPC; fully distributed; have advanced secure object system maybe based on CORBA.
BRiX: Advanced Computing Environment - Fast, no-kernel, single address space, data-centric, preemptive multithreaded, persistent, secure, multi-user operating system, with Crush function-based programming safe-language. Some code available. [Open Source, Artistic]
BRiX: SourceForge - Fast, single privilege, single address space, preemptive multithreaded, persistent, data-centric, secure, multiuser OS, with safe language, reconfigurable graphical interface.
Choices Object-Oriented Operating System - Object oriented C++ research OS.
Crush/BRiX: An Experimental Language/OS Pair - The usual variety of lively opinions, some of which are informed by study and quite insightful and useful in further understanding BRiX.
Language-based Operating Systems - Short, annotated, alphabetically sorted list and links, part of larger system.
Sting - Experimental distributed OS intended as efficient customizable substrate for modern programming languages. Base language is Scheme, but core ideas can apply to any reasonable high-level language.