From http://stackoverflow.com/questions/5528709/how-to-add-labels-to-a-plot

text_label_some_points_ggplot.R

dat <- data.frame(x = rnorm(10), y = rnorm(10), label = letters[1:10])

#Create a subset of data that you want to label. Here we label points a - e
labeled.dat <- dat[dat$label %in% letters[1:5] ,]

ggplot(dat, aes(x,y)) + geom_point() +
  geom_text(data = labeled.dat, aes(x,y, label = label), hjust = 2)

#Or add a separate layer for each point you want to label.
ggplot(dat, aes(x,y)) + geom_point() +
  geom_text(data = dat[dat$label == "c" ,], aes(x,y, label = label), hjust = 2) + 
  geom_text(data = dat[dat$label == "g" ,], aes(x,y, label = label), hjust = 2)