One alternative to R’s native data frames is the data.table package first released to CRAN in 2006. data table extends base R’s data.frame. data.table was authored principally by Arun Srinivasan and Matt Dowle.
One of the central features of the data.table was that it replaced the focus on data.frame rownames with the concept of a ‘key‘, which could be a multi-column set of values, but only one of the ‘key’ columns would operate as the index column. Further details here.
In the R ecosystem, data.table represents a different branch of the extensions to R’s basic data.frame object to that of the ‘tidyverse’ with its dplyr, readr and tibbles. The pros and cons have been discussed by their respective authors. Additionally, tidyverse’s author suggests data.table is faster for importing in some cases. See also r-bloggers.