A caution about meld, I manage a number of CentOS 6 (yeah, we're going to 7 "real soon now") and my users like to use meld, but on a few occasions it's missed some diffs. Unfortunately, I'm usually too swamped to investigate. Perhaps a current version might not have that issue.
But, I still like meld for the nice graphical interface.
There's also tkdiff which comes with the tkcvs package.