Essential list of useful R packages for data scientists

I have written couple of blog posts on R packages (here here) and this blog post is sort of a preset of all the most needed packages for data science, statistical usage and every-day usage with R. Many useful functions are available in many different R packages, many of the same functionalities also in different packages, so it all boils down to user preferences and work, that one decides to use particular package. From the perspective of a statistician and data scientist, I will cover the essential and major packages in sections. And by no means, this is not a definite list, and only a personal preference. Loading and read data into R environment is most likely one of the first steps if not the most important.