BareMetal OS

BareMetal is an open source 64-bit OS for x86-64 based computers. It is written in Assembly, and applications can be written in Assembly or C/C++. It's aimed at three target segments (High Performance Computing, Embedded Applications, and Education). It's also designed to be simple, and it's really small. Under 16Kb small. Version 0.4.8 was released recently, which includes updates to the C application library, updated documentation, and better support for SMP.