First, you can calculate the group means with aggregate:

means <- aggregate(weight ~  group, PlantGrowth, mean)

This dataset can be used with geom_text:

library(ggplot2)
ggplot(data=PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot() +
stat_summary(fun=mean, colour="darkred", geom="point",
shape=18, size=3, show.legend=FALSE) +
geom_text(data = means, aes(label = weight, y = weight + 0.08))

Here, + 0.08 is used to place the label above the point representing the mean.

An alternative version without ggplot2:

means <- aggregate(weight ~  group, PlantGrowth, mean)


boxplot(weight ~ group, PlantGrowth)
points(1:3, means$weight, col = "red")
text(1:3, means$weight + 0.08, labels = means$weight)

You can also use a function within stat_summary to calculate the mean and the hjust argument to place the text, you need a additional function but no additional data frame:

fun_mean <- function(x){
return(data.frame(y=mean(x),label=mean(x,na.rm=T)))}




ggplot(PlantGrowth,aes(x=group,y=weight)) +
geom_boxplot(aes(fill=group)) +
stat_summary(fun.y = mean, geom="point",colour="darkred", size=3) +
stat_summary(fun.data = fun_mean, geom="text", vjust=-0.7)

You can use the output value from stat_summary()

ggplot(data=PlantGrowth, aes(x=group, y=weight, fill=group))
+ geom_boxplot()
+ stat_summary(fun.y=mean, colour="darkred", geom="point", hape=18, size=3,show_guide = FALSE)
+ stat_summary(fun.y=mean, colour="red", geom="text", show_guide = FALSE,
vjust=-0.7, aes( label=round(..y.., digits=1)))

The Magrittr way

I know there is an accepted answer already, but I wanted to show one cool way to do it in single command with the help of magrittr package.

PlantGrowth %$% # open dataset and make colnames accessible with '$'
split(weight,group) %T>% # split by group and side-pipe it into boxplot
boxplot %>% # plot
lapply(mean) %>% # data from split can still be used thanks to side-pipe '%T>%'
unlist %T>% # convert to atomic and side-pipe it to points
points(pch=18)  %>% # add points for means to the boxplot
text(x=.+0.06,labels=.) # use the values to print text

This code will produce a boxplot with means printed as points and values:

I split the command on multiple lines so I can comment on what each part does, but it can also be entered as a oneliner. You can learn more about this in my gist.