Despite the ggplot2 project — the most popular data visualization package for R — being in maintenance mode, RStudio's Hadley Wickham has given the R community a surprise gift with a version 2.0.0 update for ggplot2. According to Hadley this is a "huge" update with more than 100 fixes and improvements.
The most significant addition is that ggplot2 now has a formal extension mechanism, which means that package authors can now create their own geoms (plot types), statistics (data aggregation/transformation methods) and themes. There are also a number of smaller improvements, including making it easy to draw curved lines between points with geom_curve, a way to suppress overlapping text labels, and a way to add labels with rounded enclosing boxes to plots. There are also a few minor changes to the appearance of plots, with some changes to colors and text sizes to improve readability and (my personal favorite) the elimination of the diagonal line that used to appear in the color boxes in legends.
If you're new to ggplot2, getting to grips with the Grammar of Graphics system can be a steep learning curve, but it's well worth it in terms of long-term productivity in creating beautiful graphics with R. Hadley Wickham's book, Elegant Graphics for Data Analysis, is a great place to get started. Experienced ggplot2 users will also appreciate the freshly-updated ggplot2 cheatsheet from the RStudio team.
This new update is mostly compatible with older versions of ggplot2, but it may require updates to existing code in some cases. (If you want to try the new ggplot2 but still have access to old versions for compatibility, take a look at the checkpoint package.) The updated ggplot2 package is now available for download via CRAN with install.packages("ggplot2"), or via the ggplot2 Github repository.
RStudio Blog: ggplot2 2.0.0