In scatter plots with large amounts of data which results in lots of overplotting, it can be useful to indicate density of data points. Smoothscatter does this with a color palette. However, smoothscatter uses R base graphics. With ggplot2, a similar effect can be achieved using the densCols function and scale_color_identity:

x <- rnorm(20000)
y <- x + rnorm(20000, 0.05)
df <- data.frame(x = x, y = y,
  d = densCols(x, y, colramp = colorRampPalette(rev(rainbow(10, end = 4/6)))))
p <- ggplot(df) +
    geom_point(aes(x, y, col = d), size = 1) +
    scale_color_identity() +
    theme_bw()
print(p)

Alternatively, one can introduce transparency, which can (for low densities) be helpful for overplotting. Or use geom_hexbin:

p <- ggplot(df) + geom_hex(aes(x, y), bins = 100) +
         scale_fill_gradientn("", colours = rev(rainbow(10, end = 4/6)))
print(p)