summarise(group_by(mtcars, cyl),
          mean = mean(disp))
      mean
1 230.7219

ddply(mtcars, .(cyl), summarise,
       mean = mean(disp))
  cyl     mean
1   4 105.1364
2   6 183.3143
3   8 353.1000

sessionInfo()
R version 3.0.2 (2013-09-25)
Platform: x86_64-w64-mingw32/x64 (64-bit)
dplyr_0.1.2
plyr_1.8