Recs.
Updated
SpecsUpdate
Pros
Pro Supports 3 way merges with common ancestor
For modern version control systems, 3way merge support is a basic requirement, but many other open source diff viewers do not adequately handle 3way merges. KDiff uses a 4 panel system, showing Left, Right, Base (very important) at the bottom. The Base (common ancestor) panel is extremely helpful for complex merges where both Left and Right diverged significantly from their common ancestor.
Cons
Con Confusing GUI
4 sub-windows (when you really only need 3), a lot of different colors and even more confusing result-window. No links what has changed between versions and and the result. It clearly shows it's dated or rather outdated. Great if you ever need to do a command line merge, otherwise it sucks.
Con Has problems with fonts
When highlighting, Fixed-size fonts and tabs are shown incorrectly, misaligned, and overwriting the text. Experienced on Ubuntu14/16, and quite hard to fix. Setting a different fixed-size font makes the problem. Can be a problem with the Gnome/KDE compatibility.