Place a border around points

It's a bit obscure, but you have to use pch>20 (I think 21:25 are the relevant shapes): fill controls the interior colo(u)ring and colour controls the line around the edge.

(g0 <- ggplot(df, aes(x=x, y=y))+geom_point(aes(fill=id), 
       colour="black",pch=21, size=5))

update: with recent ggplot2 versions (e.g. 2.0.0, don't know how far back it goes) the default guide is a colourbar. Need g0 + guides(fill="legend") to get a legend with points as in the plot shown here. The default breaks have changed, too: to exactly replicate this plot you need g0 + scale_fill_continuous(guide="legend",breaks=seq(0.2,0.8,by=0.1)) ...

Related but not identical: how to create a plot with customized points in R? . The accepted answer to that question uses the layering technique shown in @joran's answer, but (IMO) the answer by @jbaums, which uses the pch=21 technique, is superior. (I think shape=21 is an alternative, and perhaps even preferred, to pch=21.)

PS you should put colour outside the mapping (aes bit) if you want to set it absolutely and not according to the value of some variable ...

The first question's a gimme:

ggplot(df, aes(x=x, y=y)) + 
    geom_point(aes(colour=id), size=12) + 
    geom_point(shape = 1,size = 12,colour = "black")

And, oh, you don't want an extra legend. I think that does it then: