MEDDAC is the seeding point for designing a new approach to Operating System design itself. It is an Open Source project with roots in mainframe design and development the author began in 1979. Monitor Editor Disassembler Decompiler Assembler Compiler
The FDOS Operaing System is 100% written in Assembler (NASM). It supports FAT 12, TSR and a small Multi Kernel System. The source is very good documentated (only in English). A German, russian and English Documentation (about 40 pages) are included.
Horizon is a research project aimed at developing a 32-bit Operating System for the Intel architecture. It's written in C and assembler (GAS syntax) and one of its main characteristics is full network integration.