I have been used to Araxis merge for doing file/folder diff and also help in merging. (replace, add before, add after, etc.)
Was searching for similar tool on Linux. Found that Kdiff can be a good option. http://kdiff3.sourceforge.net/
Dowloaded Kdiff source and followed instructions to build and install it on my SuSe 9.2
I had a problem while compiling translation software ( which it tries to install by default). Skipped that folder and Kdiff was built and installed successfully.
Great, it has 3 view diff. In addition some good basic merge features, though not as good as Araxis.