How to summarise data by group with weighted mean?

As suggested on an old R thread, you can use by instead:

wt <- c(5,  5,  4,  1)/15
x <- c(3.7,3.3,3.5,2.8)
xx <- data.frame(avg=x, value=gl(2,2), weight=wt)
by(xx, xx$value, function(x) weighted.mean(x$avg, x$weight))

This being a 'million ways to skin a cat' question, here's a plyr solution (using @chl's example data):

ddply(xx,.(value),summarise, wm = weighted.mean(avg,weight))