HelenOS features in a single screenshot. The image depicts the HelenOS compositing GUI, networking, filesystems, sound subsystem and a multithreaded, multiprocessor 64-bit kernel in action. The Colorful Prague picture used in the screenshot is a courtesy of Miroslav Petrasko.
HelenOS is a portable microkernel-based multiserver operating system designed and implemented from scratch. It decomposes key operating system functionality such as file systems, networking, device drivers and graphical user interface into a collection of fine-grained user space components that interact with each other via message passing. A failure or crash of one component does not directly harm others. HelenOS is therefore flexible, modular, extensible, fault tolerant and easy to understand.
More info at the HelenOS webpage.