Extensible

BITS
The Component Based Operating System: based on describing system resources as independent components, lets applications implement their own abstractions, define their own protection schemes, participate in resource management.
http://www.soi.city.ac.uk/~patty/bits.html

A Caching Model of Operating System Kernel Functionality
Stanford Cache Kernel, supervisor-mode component of V++ OS; caches system objects (threads, address spaces) to raise performance; microkernel alternative, performance equals normal monolithic OSs, yet gives application-level control of system resources, more modularity, scalability, smaller size, means of fault containment.
http://www-dsg.stanford.edu/papers/cachekernel/main.html

Extensible Operating Systems
Brief description, and on-site links to descriptions of Choices, Exokernel, GLUnix, VINO, SPIN.
http://www.cs.unm.edu/~riesen/prop/node26.html

SPIN
Dynamically extensible, Exokernel-based, provides many core services: scheduler, kernel threads, domains, event dispatcher, security mechanisms, primitive VM operations. Blurs distinction between kernels and applications, which traditionally live in user-level address spaces, separated from kernel resources and services by an expensive protection boundary. Lets applications specialize the kernel by dynamically linking new code into running systems.
http://www.cs.washington.edu/research/projects/spin/www/

Using Kernel Extensions to Decrease the Latency of User-Level Communication Primitives
Suggests solving networking and distributed systems latency via operating system extensibility; University of New Mexico Technical Report.
http://www.cs.unm.edu/~riesen/prop/