Build your own (insert technology here) when you know nothing about it. Dozens of links to curated tutorials about lots of different things, from 3D rendering to how network stacks work to other stuff.
The full text of _The Handbook of Applied Cryptography_, one of the best textbooks available in the field. The original authors have gotten permission from the publisher to put the text online as sets of PostScript and .pdf files for anyone to download and share.
A grassroots peer-based education project based around online learning and remote collaboration between small groups of students. They just started up so things are a little thin at the moment. This is an interesting experiment to keep your eye on. Creative Commons.
KA Lite is the offline version of the Khan Academy instructional materials, i.e., it's packaged in such a way that it can be stored on and accessed from flash or optical media. You don't need net.access to get to it. Includes documentation and materials for teachers and administrators in addition to the educational materials. Even includes its own web server (requires Python) if you don't have one installed already.
A massive list of free-as-in-beer ebooks about programming: Languages, platforms, tools, and more. Ideal for teaching classes, continuing education, reference, and getting a jump start on programming in school.
RACHEL - Remote Area Community Hotspot for Education and Learning. An opensource project for a raspi that serves an offline libary of educational material for bandwidth poor locations. education Includes what they call "utility content," which seems to be reference material. OER - Open Educational Resources. Internationalized. software archive
A FOSS textbook that teaches you how to write an OS from scratch. Reading hardware datasheets and official documentation, system organization, very little sample code to get you solving problems on your own. How to debug on the bare metal using gdb and QEMU. Requires knowledge of C and ideally the Linux environment. Has a reference to a sample OS which illustrates certain concepts.
A wiki as well as a meta wiki that tries to unify different sources of information for the purposes of scholarship and easy citation. Claims that it goes back to primary sources for its constructions. Offers fact maps and topic trees, among other knowledge structures.