Amoeba is a distributed system that runs on a few dozen machines on the same LAN. Amoeba is designed to make the complete system be transparent and look like a single timesharing system.
Globe is a distributed system designed to run worldwide with a billion users and a trillion (possibly mobile) objects. It addresses the issues of scalability, replication, security, etc. in very large distributed systems.
Paramecium is a single machine operating system targeted at embedded systems. It is designed for flexibility, and allows modules to be run in either kernel space or user space, as is required, and do so in a secure way.