Boxplot in R showing the mean

With ggplot2:

p<-qplot(spray,count,data=InsectSprays,geom='boxplot')
p<-p+stat_summary(fun.y=mean,shape=1,col='red',geom='point')
print(p)

abline(h=mean(x))

for a horizontal line (use v instead of h for vertical if you orient your boxplot horizontally), or

points(mean(x))

for a point. Use the parameter pch to change the symbol. You may want to colour them to improve visibility too.

Note that these are called after you have drawn the boxplot.

If you are using the formula interface, you would have to construct the vector of means. For example, taking the first example from ?boxplot:

boxplot(count ~ spray, data = InsectSprays, col = "lightgray")
means <- tapply(InsectSprays$count,InsectSprays$spray,mean)
points(means,col="red",pch=18)

If your data contains missing values, you might want to replace the last argument of the tapply function with function(x) mean(x,na.rm=T)

Check chart.Boxplot from package PerformanceAnalytics. It lets you define the symbol to use for the mean of the distribution.

By default, the chart.Boxplot(data) command adds the mean as a red circle and the median as a black line.

Here is the output with sample data; MWE:

#install.packages(PerformanceAnalytics)    
library(PerformanceAnalytics)
chart.Boxplot(cars$speed)