The doParallel and foreach packages have made my life so much easier by allowing me to parallelize my code and run it on a compute-optimized instance on Amazon EC2! I use them very often. But that would not have been possible without the RStudio AMIs released by Louis Aslett. Finally, I have to mention the stringr package which really makes working with strings a walk in the park. Use it in every text mining application. And I also use knitr very frequently to produce high quality reports of my work. Many thanks for this amazing package Yihui Xie!