Single-Addres Space Operating Systems

Angel (City University of London)
Angel is designed as a generic parallel and distributed operating system, although it is currently targeted towards a high-speed network of PCs. This model of computing has the dual advantage of both a cheap initial cost and also a low incremental cost. By treating a network of nodes as a single shared memory machine, using distributed virtual shared memory (DVSM) techniques, we have addressed both the needs for improved performance and provided a more portable and useful platform for our applications.
Mungi (University of New South Wales)
A new operating system based on a single, flat virtual address space, orthogonal persistence, and a strong but unintrusive protection model.
Nemesis (University of Cambridge, UK)
Pegasus has produced an entirely new operating system, whose design is geared to support of high-performance applications which require a consistent quality of service (QoS), such as those which use multimedia. This operating system, called Nemesis, currently runs on a number of platforms including Intel, Alpha (21064 and 21164) and StrongARM.
Nemesis is a single-address-space system, with an extremely light-weight kernel ('Nemesis Trusted Supervisor Code', or NTSC), and strong emphasis on performing operating system functions in the user's domain (thus avoiding the need for expensive protection regime changes).
Opal (University of Washington)
Related to: Mach
Opal is a single-address-space operating system for 64-bit architectures. All Opal threads execute with a single global address space. The existence of a single address space simplifies sharing of complex (pointer-rich) data among cooperating applications, as well as persistent storage of that data, because pointers have the same meaning to all threads for potentially all time. Opal provides protection in the single address space; each thread executes within a protection domain that defines which pages it has the right to access.
Sombrero (Arizona State University)