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

ggtech

https://github.com/ricardo-bion/ggtech

ggplot2 tech themes, scales, and geoms.

# Example from https://github.com/ricardo-bion/ggtech
library(ggplot2)
library(ggtech)

# Make sure to install the required fonts 
# (instructions at the end of this file).
extrafont::font_import(pattern = 'Guardian-EgypTT-Text-Regular.ttf', prompt=FALSE)

d <- qplot(carat, data = diamonds[diamonds$color %in%LETTERS[4:7], ], geom = "histogram", bins=30, fill = color)

Tech themes and scales:

d + theme_tech(theme="airbnb") + ggtitle("Airbnb ggplot2 theme") + scale_fill_tech(theme="airbnb")

d + theme_tech(theme="etsy") + ggtitle("Etsy ggplot2 theme") + scale_fill_tech(theme="etsy")

d + theme_tech(theme="facebook") + ggtitle("Facebook ggplot2 theme") + scale_fill_tech(theme="facebook")

d + theme_tech(theme="google") + ggtitle("Google ggplot2 theme") + scale_fill_tech(theme="google")

d + theme_tech(theme="twitter") + ggtitle("Twitter ggplot2 theme") + scale_fill_tech(theme="twitter")

Tech geoms, inspired by emoGG.

d2 <- data.frame(x = c(1:4, 3:1), y=1:7)
ggplot(aes(x,y), data=d2) + 
  geom_tech(size=0.25, theme="airbnb") + 
  theme_tech("airbnb") +
  ggtitle("Airbnb geom")

ggplot(aes(x,y), data=d2) + 
  geom_tech(size=0.15, theme="etsy") + 
  theme_tech("etsy")+
  ggtitle("Etsy geom")

ggplot(aes(x,y), data=d2) + 
  geom_tech(size=0.15, theme="facebook") + 
  theme_tech("facebook")+
  ggtitle("Facebook geom")

ggplot(aes(x,y), data=d2) + 
  geom_tech(size=0.25, theme="google") + 
  theme_tech("google" ) +
  ggtitle("Google geom")

ggplot(aes(x,y), data=d2) + 
  geom_tech(size=0.15, theme="twitter") + 
  theme_tech("twitter") +
  ggtitle("Twitter geom")

Install fonts:

You have to install the necessary fonts manually before using ggtech. Mofidy the destfile if you are using Windows or Unix.

## Facebook 
download.file("http://social-fonts.com/assets/fonts/facebook-letter-faces/facebook-letter-faces.ttf", "/Library/Fonts/facebook-letter-faces.ttf", method="curl")

font_import(pattern = 'facebook-letter-faces.ttf', prompt=FALSE)
font_import(pattern = 'Lucida', prompt=FALSE)


## Google 
download.file("http://social-fonts.com/assets/fonts/product-sans/product-sans.ttf", "/Library/Fonts/product-sans.ttf", method="curl")

font_import(pattern = 'product-sans.ttf', prompt=FALSE)
font_import(pattern = 'Roboto', prompt=FALSE)


## Airbnb 
download.file("https://dl.dropboxusercontent.com/u/2364714/airbnb_ttf_fonts/Circular%20Air-Medium%203.46.45%20PM.ttf", "/Library/Fonts/Circular Air-Medium 3.46.45 PM.ttf", method="curl")

download.file("https://dl.dropboxusercontent.com/u/2364714/airbnb_ttf_fonts/Circular%20Air-Bold%203.46.45%20PM.ttf", "/Library/Fonts/Circular Air-Bold 3.46.45 PM.ttf", method="curl")

font_import(pattern = 'Circular', prompt=FALSE)


## Etsy 
download.file("https://www.etsy.com/assets/type/Guardian-EgypTT-Text-Regular.ttf", "/Library/Fonts/Guardian-EgypTT-Text-Regular.ttf", method="curl")

font_import(pattern = 'Guardian-EgypTT-Text-Regular.ttf', prompt=FALSE)


## Twitter 
download.file("http://social-fonts.com/assets/fonts/pico-black/pico-black.ttf", "/Library/Fonts/pico-black.ttf", method="curl")

download.file("http://social-fonts.com/assets/fonts/arista-light/arista-light.ttf", "/Library/Fonts/arista-light.ttf", method="curl")

font_import(pattern = 'pico-black.ttf', prompt=FALSE)
font_import(pattern = 'arista-light.ttf', prompt=FALSE)