I thought I had to enter “diff” in the diff tool field, but I was wrong: it didn’t work. Apparently RapidSVN expects a graphical application instead of a text-mode application that runs on a terminal.
But fear not! There’s an application for this. It’s called meld.
So…
sudo apt-get install meldAnd in RapidSVN → Preferences → Programs → Diff Tool simply enter meld. Now you should be able to right click a file and select the Diff… flavour you prefer (to Base, to HEAD or to something else), and get the diff between the versions!
You might remember I always say I tend to use the command line for dealing with SVN, but sometimes I feel lazy (specially when dealing with many files at the same time), and RapidSVN is very handy to get a quick overview of what’s changed. Besides, it doesn’t break randomly like Subclipse/Subversive whenever Eclipse is updated, and can be used outside Eclipse, for non-Eclipse projects. Thanks to mrdoob for the cool app recommendation :)

eyebex
People like me who like the concept of “virtual padding lines” better than those “merge lines” in the pane between the files should take a look at Diffuse [1] instead ;-)
[1] http://diffuse.sourceforge.net/
sole
Oh, good to know! Thanks :)
Uditha Madumal
Thanks. I had done it before using copy-paste the binaries & giving path to bin inside RapidSvn. But failed this time. So I Installed using apt-get & just mentioned “meld” in “RapidSvn” It worked….