Q:

How to force a legend inside a plot to be horizontal and not vertical?

I'd like the legend inside the plot to be horizontal (each one is beside each other not one is above the other) 
Is there a way to change from vertical to horizontal?
Example:
    mtcars$cyl <- factor(mtcars$cyl, labels=c("four","six","eight"))
    ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
   opts(legend.position = c(0, 1), title="Legend placement makes me sad")

A:

You get horizontal legend by adding argument legend.direction="horizontal" inside the theme() or opts()
theme(legend.direction="horizontal")

Older versions
opts(legend.direction="horizontal")

A:

When you create your legend, you can use the argument 'horiz' which is automatically at FALSE 
legend( ... , horiz=TRUE)

