stevethemes is an R package including various ggplot2 themes and theme elements that I find useful and aesthetically pleasing. It also includes some various helper functions for adjusting plots to taste for either the web or a printed manuscript. Its use heavily implies, but does not formally require, free fonts you can download from Google. In particular, it suggests use of Titillium Web, Open Sans, Fira Sans, and Crimson Pro. The package has simple forms of these fonts available for you to install on your computer, though you can easily download these fonts (and more) from Google as well.
install.packages("stevethemes")
The development version may also have some extra goodies not yet published on CRAN. You can install the development version of stevethemes from Github via the devtools package. I suppose using the remotes package would work as well.
devtools::install_github("svmiller/stevethemes")