Here are links I find useful. Hope this can help. Tell me if some of them are dead.
- KernelTrap For those who wants to know how the Linux kernel development is going. There's a lot of informations about the technical details too. Very interesting.
- ArsTechnica You'll find here (of course) news, very interesting technical articles. (the latest are about the x86/64 intrinsics and the PPC for example), tips & tweaks.
- RootPrompt.org Interesting articles on technical subjects. Very interesting.
- The APOD This is not really a news site. You'll find here pictures of stellar objects (one picture a day), with the comment of a professional astronom.
- The C++ FAQ Lite this is a very complete FAQ about the C++ language.
- Thinking in ... These are free-to-download books by Bruce Eckel. The Thinking in C++ is very good. Also exist Thinking in Patterns and Thinking in Java.
- Advanced Bash Scripting A very good documentation for people who really want to write bash scripts
- Vectorized assembly instructions These are docs about MMX/MMX2/SSE/whatever. This is the only doc I know about this kind of assembly (of course, the Intel & AMD docs excluded). If you know one that is better, please let me know.