A complete textbook on computer architecture and assembly language programming, as a website, in easy-to-digest pages.
Nyuzi is an experimental GPGPU processor hardware design focused on compute intensive tasks. It is optimized for use cases like deep learning and image processing.
This project includes a synthesizable hardware design written in System Verilog, an instruction set emulator, an LLVM based C/C++ compiler, software libraries, and tests. It can be used to experiment with microarchitectural and instruction set design tradeoffs.
The Nand Game will take you though building a working computer, starting from the most basic components. It does not require any prerequisites, in particular it does not require any previous knowledge about computer architecture or software, and does not require math skills beyond addition and subtraction. It does require some patience - some of the tasks may take a while to solve.
Twelve simple truths (and a couple of corollaries) about networks, networking, and network protocols.
Wikihouse is an archive of open source building designs contributed by engineers and architects around the world. Each design is peer-reviewed and analyzed by others to ensure that the designs are sound. Some of them have already been built and the results posted for further review. The design principles include being as easy to build by people with minimal formal training, as energy efficient as possible, and easy to modify to fit a particular purpose. All designs are made in Google Sketchup and can be cut with an automill if available.
A pair of books licensed in the Creative Commons (BY) which talk about the architecture of open source software. The core developers of four dozen projects talk about how their software is structured, and most importantly why. They were written so that F/OSS developers wouldn't have to learn by reinventing the wheel, instead there would be a reference.
There are two books, which you can read online and download for free. Or you can buy them as Kindle editions (the proceeds go to Amnesty International).
The J2 is a clean-room designed FOSS processor core and SOC that implements the SuperH instruction set. hardware VHDL BSD license. Royalty and patent free. Compile the VHDL into a bitstream and upload it into an FPGA. Known to run Linux. Hybrid RISC architecture, fixed-length 16-bit instructions, 32-bi registers and address space. Has no MMU for now but it's in the works due to patent issues. Cheap to manufacture (roughly $0.03/processor). security audited.
An online book that talks about the linux kernel, how it boots up, and the theory and practice behind the subsystems.