DeepHack: Spelunking for the 21st Century
QEMU is a generic and open source machine emulator and virtualizer.
- More oscilloscope fun
The Elements of Computing Systems
MMIX is a 64-bit RISC machine used in Volume 4A of Donald Knuth's Art of Computer Programming and it will eventually be used in all other volumes of that series.
Open Computation Hardware
The OR1200 is an open-source synthesizable CPU.
What Every Programmer Should Know about Computer Memory
A 7-part series by Ulrich Drepper (complete PDF):
CorePy: Assembly Programming from Python
A Plumber's Wish List for Linux
Looking for a useful systems software project? Version 2 of the list by Kay Sievers (20 Oct 2011).
Original Unix Shell
Unix V6 Shell by Ken Thompson (900 lines)
0.01 release by Linus Torvalds (10,000 lines).