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.
In their own words: "DevDocs combines multiple API documentations in a fast, organized, and searchable interface." My words: DevDocs brings together the docs of over fifty (and growing) libraries and languages. You choose which you are interested in and you searches are limited to those topics. Your choice of interests are remembered the next time you use it. Wide ranging topics from HTML, CSS and JS to C, C++ and PostgreSQL to NodeJS, Knockout and jQuery. It also supports keyboard shortcuts and an offline mode which is handy for coding on planes. You can even vote on new topics; see their About page for details.
Mozilla Developer Network https://developer.mozilla.org/en-US/
for general web technologies
In their words: "The Mozilla Developer Network (MDN) is an evolving learning platform for Web technologies and the software that powers the Web" My words: While some of the Mozilla Developer Network, or more commonly the MDN, is about Mozilla specific technologies like Firefox OS, they mostly maintain an amazing body of in depth information on the open web technologies, i.e. anything that happens in a browser, and they cover the details for all the major browsers including what versions support what. For in depth info on the client-side this is my go to source.
Can I Use http://caniuse.com/
In their words: "Provides up-to-date browser support tables for support of
front-end web technologies on desktop and mobile web browsers." My words: If you just need to know which browsers (make and version) support which features, this is by far the quickest place to get that information.
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.