raster package: Lines around each cell

You can use rasterToPolygons:

plot(r, col=c(topo.colors(200)), axes=FALSE, box=FALSE)
plot(rasterToPolygons(r), add=TRUE, border='black', lwd=1) 

If you are open to using ggplot2, I offer a possible solution using geom_tile(). Pixels can be outlined by using the colour argument to geom_tile. The drawback is that your data may need some re-formatting to use with ggplot2.

library(ggplot2)
library(reshape2)

dat = melt(volcano[26:40, 26:40])

p = ggplot(dat, aes(x=Var1, y=Var2, fill=value)) +
    geom_tile(colour="grey20") +
    scale_fill_gradientn(colours = terrain.colors(10))

ggsave("tile_plot.png", plot=p, height=6, width=7, dpi=150)