Three things I like about stackoverflow (and stackexchange in general): the license currently used (cc by-sa 3.0 with attribution required), the possibility to download the databases of all stackexchange websites and the abscence of registrationwall.

Interesting documents related to the C language:

• Understanding Integer Overflow in C/C++
• What every C programmer should know about undefined behavior

Interesting documents related to software licensing:

• SQLite page on copyright

Interesting documents related to memory, caches:

• What every programmer should know about memory

Interesting documents related to floats:

• What every programmer should know about floating-point arithmetic

Related to structure packing:

• The Lost Art of Structure Packing