This should work:
p <- ggplot(df, aes(x=rating, fill=cond)) +
geom_density(alpha=.3) +
xlab("NEW RATING TITLE") +
ylab("NEW DENSITY TITLE")
p <- p + guides(fill=guide_legend(title="New Legend Title"))
(or alternatively)
p + scale_fill_discrete(name = "New Legend Title")