How can I remove the legend title in ggplot2?

The only way worked for me was using legend.title = theme_blank() and I think it is the most convenient variant in comparison to labs(fill="") and scale_fill_discrete(""), which also could be useful in some cases.

ggplot(carrots,aes(y=MeanLength,x=Farm,fill=Type)) + 
geom_bar(position="dodge") +
opts(
    legend.position="top",
    legend.direction="horizontal",
    legend.title = theme_blank()
)

P.S. There are more useful options in documentation.

You can modify the legend title by passing it as the first parameter to a scale. For example:

ggplot(carrots, aes(y=MeanLength, x=Farm, fill=Type)) + 
  geom_bar(position="dodge") +
  theme(legend.position="top", legend.direction="horizontal") +
  scale_fill_discrete("")

There is also a shortcut for this, i.e. labs(fill="")

Since your legend is at the top of the chart, you may also wish to modify the legend orientation. You can do this using opts(legend.direction="horizontal").

I found that the best option is to use + theme(legend.title = element_blank()) as user "gkcn" noted.

For me (on 03/26/15) using the previously suggested labs(fill="") and scale_fill_discrete("") remove one title, only to add in another legend, which is not useful.

You can use labs:

p + labs(fill="")