FreeOS, The Resource Center for Free Operating Systems
GEM, one of the first windowing systems available for the PC
Inferno: a compact operating system for building cross-platform distributed systems
ITRON: a Japanese standard for embedded operating systems already used by more than three billion microprocessors found in mobile phones, digital cameras, CD players, and many other electronic devices including even satellites
OpenMosix: a Linux kernel extension for single-system image clustering. This kernel extension turns a network of ordinary IA-32 computers into a supercomputer for Linux applications.
papers
C10K Problem, an interesting paper about improving the performance and scaling of servers on Linux and other Unix-type operating systems
Zero Install: makes software installation not merely easy, but unnecessary. Users run their applications directly from the Internet from the software author's pages.
Nemesis, whose design is geared to the support of time-sensitive applications requiring a consistent Quality of Service (QoS), such as those which use multimedia. Nemesis provides fine-grained guaranteed levels of all system resources including CPU, memory, network bandwidth and disk bandwidth.