Amoeba
Powerful free, open source, microkernel-based system that turns a collection of workstations or single-board computers into a transparent distributed system. Used in academia, industry, and government for many years. Runs on many platforms.

Amoeba Distributed Operating System
Short page of Amoeba links.

Amoeba Distributed Operating System
Technical paper, describes Amoeba, with special focus on its object-based design and use of capabilities, and its innovative Wide Area Network communication scheme; ends by comparing Amoeba and the Sprite distributed OS.

Bibliography on the Distributed Operating System Amoeba
Part of the Computer Science Bibliographies: search, browsing, statistics.