ggplot2 is a plotting system for R, based on the grammar of graphics, which tries to take the good parts of base and lattice graphics and avoid bad parts. It takes care of many of the fiddly details that make plotting a hassle (like drawing legends) as well as providing a powerful model of graphics that makes it easy to produce complex multi-layered graphics.
To install or update, run:
install.packages(c("ggplot2", "plyr"))
Find out more at http://had.co.nz/ggplot2, and check out the nearly 500 examples of ggplot in use. If you're interested, you can also sign up to the ggplot2 mailing list at http://groups.google.com/group/ggplot2, or track development at http://github.com/hadley/ggplot2
To install the development version of ggplot2, it's easiest to use the devtools package:
# install.packages("devtools")
library(devtools)
install_github("ggplot2")