ggplot2 now has an official extension mechanism. This means that others can now easily create their own stats, geoms and positions, and provide them in other packages. This should allow the ggplot2 community to flourish, even as less development work happens in ggplot2 itself. This page showcases these extensions.
ggiraph

Make ggplot interactive
ggstance

Horizontal versions of ggplot2 geoms
ggalt

Extra coordinate systems, geoms & stats
ggforce

Accelarating ggplot2
ggrepel

Repel overlapping text labels
ggraph

Plot graph-like data structures
ggpmisc

Miscellaneous extensions to ggplot2
geomnet

Network visualizations in ggplot2
ggExtra

Marginal density plots or histograms
gganimate

Create easy animations with ggplot2
plotROC

Interactive ROC plots
ggthemes

ggplot themes and scales
ggspectra

Extensions for radiation spectra
ggnetwork

Geoms to plot networks with ggplot2
ggtech

ggplot2 tech themes, scales, and geoms
ggradar

radar charts with ggplot2
ggTimeSeries

Time series visualisations
ggtree

A phylogenetic tree viewer
ggseas

Seasonal adjustment on the fly

ggthemes

https://github.com/jrnold/ggthemes

Some extra geoms, scales, and themes for ggplot

# Example from https://github.com/jrnold/ggthemes
library(ggplot2)
library(ggthemes)

p2 <- ggplot(mtcars, aes(x = wt, y = mpg, colour = factor(gear))) +
  geom_point() +
  ggtitle("Cars")

# Economist theme
# A theme that approximates the style of plots in The Economist magazine.
p2 + theme_economist() + scale_colour_economist() 

# Stata theme
# Themes and scales based on the graph schemes in Stata.
p2 + theme_stata() + scale_colour_stata()

# Fivethirtyeight theme
# Theme and color palette based on the plots at fivethirtyeight.com.
p2 + geom_smooth(method = "lm", se = FALSE) +
  scale_color_fivethirtyeight("cyl") +
  theme_fivethirtyeight()

# Tableau Scales
# Color, fill, and shape scales based on those used in the Tableau software.
p2 + theme_igray() + scale_colour_tableau()