SO-Excel
Every now and then I stumble over excellent answers on StackOverflow. Of course I could star them (to be precise, not the answer but rather it's question is starred). However, I feel there is a need to collect these awesome sources. Maybe this becomes a book at some day, who knows.
Answers will be copied from the website, probably some editing is required to fix typos, layout and formatting. Questions, however, will probably not be copied. Most very good questions usually make sense without the context of a question, in the other cases some explanation is probably preferable.
Answers on SO are licensed by SO under the CC BY-SA 3.0 license.
Idea
Text books are usually written such that a person not knowing the topic before, understands the topic after carefully reading that very book. The problem is begind the word carefully. It happens surprisingly often, that not only one particular, but many (if not all) text books on a topic fail to explicitely explain a crucial concept. This is kind of a self-fulfilling prophecy effect: Maybe the concept is almost obvious, hence no text book explains it, hence it becomes mystical among students. The students wonder why this mystical concept is never explained and the authors wonder why students make such a big deal out of that obvious concept.
I think this is where the Q/A-format excels. It is certainly not complete in any aspect, but you will presumably learn something (you'd never wanted to know) while browsing it. And browsing Q/A is much more fun than browsing text books since the writing style is much more compact. Ideally, you learn the principal concept (i.e., the idea) in an answer and then, if you want to, look up the details in your favourite text book.
TODO
A list of answers which may qualify for this collection