Rather I have multiple sources; I don't usually like to use such a service if there is an alternative which can be easily made offline as PDF, HTML or at best,

Rather I have multiple sources; I don't usually like to use such a service if there is an alternative which can be easily made offline as PDF, HTML or at best, EPUB. And better not require an account.. There are only few good ones on Wikibooks, still they are worth it... Eg: those on C Programming... You can get some decent sources from university sites... I think I can learn things very quickly from lecture slides + bit of experimentation. Since almost all things I want to learn are open source, there are always some great resources in form of documentation for me. I often like terse documentation rather than tutorials since it facilitates faster learning if you know how;