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.