Haiku
A modern, fast, powerful desktop operating system inspired by BeOS.
MikeOS
A simple 16-bit operating system for x86 PCs, written in assembly language and intended for educational use. I learned x86 assembly by studying its code, so I recommend it to anyone who wants to learn x86 assembly.
OS/2 Museum
Blog by Michael Necasek on the history of OS/2 and random aspects of PC software and hardware history, with emphasis on the 1987-1997 period.
pagetable.com
Interesting bits of computing history.
Planet Virtual Boy
A Web site dedicated to a unique yet obscure 3D video game console, the Nintendo Virtual Boy.