An introduction to R appropriate for a complete beginner, by Trevor Martin at Princeton University.
Loading data into R
Examples from a variety of common file/data types.
A manual from cran-R, which is much more expansive and detailed, this is better from someone looking for depth or troubleshooting loading data.
Installing and loading libraries
A detailed introductory guide by MarinStatsLectures, Mike Marin and Ladan Hamadani.
Converting from wide to long data frame format in R
Gives examples for gather(), spread(), melt() and others as ways to combine or separate rows/columns of data.
Looking at and Manipulating Data in R
A short example of how to make and call values from a data frame.
Slides by Dr. Denise Ferrari with a broad introduction to data in R, includes basic examples. (UCLA)
An index of links to data manipulation tutorials with examples.
Explanation of several methods for aggregating and summarizing data, with examples.
One detailed example, posted by Dr. Dave Tang.
Describes options for the aggregate function, with several basic examples.
Quick tutorials with examples for a variety of plots, assumes a small amount of previous R knowledge
This tutorial shows how to create histograms, box plots, and scatter plots (as well as a couple other plots).
Creating functions in R
A code-based example that assumes some prior knowledge. Better if you prefer to work through code rather than reading text. (Software Carpentry)
A text-based guide to functions in R. (Dr. Rich FitzJohn and Dr. Daniel Falster)
Linear models in R
A good step by step example of regression that also has some basic R introduction material. (Harvard)
An extensive tutorial with a mix of code and explanations, this also has a sequel covering mixed effects models. (Dr. Bodo Winter)
Introductory R manual by cran-R.