There are far too many fine point details in day to day software development to keep only in my head. Here are the sources of documentation I use on a weekly basis. Note; given the majority of the work I do is WebDev using ASP.NET these sources are skewed in that direction.
Mozilla Developer Network https://developer.mozilla.org/en-US/ or https://developer.mozilla.org/en-US/docs/Web for general web technologies
Can I Use http://caniuse.com/
Well duh. The ten thousand pound gorilla of software development question and answer sites. I think most people still get there from a search engine result, but never the less it is the Godzilla of dev answers. Here is a useful tip regarding StackOverflow: if you don't find the answer to your problem, slow down and go through the process of asking the question on the site in a careful way. 80% of the time when I do this I figure out the issue in the process. I also have a request; take a moment and create an account (most people use the site anonymously) and Up-Vote questions and answers you find useful. Personally I love the feeling of getting an Up-Vote.