dplyr

So useful!

http://cran.rstudio.com/web/packages/dplyr/vignettes/introduction.html

Hadley explains (at the above link):

The dplyr package makes each of these [data processing] steps as fast and easy as possible by:

• Elucidating the most common data manipulation operations, so that your options are helpfully constrained when thinking about how to tackle a problem.
• Providing simple functions that correspond to the most common data manipulation verbs, so that you can easily translate your thoughts into code.
• Using efficient data storage backends, so that you spend as little time waiting for the computer as possible.

The dplyr debut blog post may also be of interest.