Meld is a nice visual diff and merge tool. With Meld you can compare files and reconcile differences. It is written in Python and uses GTK+ as a GUI toolkit.
In this guide we will download and run Meld from source. Meld's source code is kept in a Git repository hosted by the GNOME project. Meld is actually pretty easy to run without installing it, but I had trouble getting the latest versions running on OS X recently. This is a problem because I've been trying to keep the Meld port current for MacPorts.
To get started clone the repo and try running Meld.
$ git clone https://git.gnome.org/browse/meld $ bin/meld
If you're on Linux this will probably just work. If you're on a Mac you may get some errors like the following:
Couldn't bind the translation domain. Some translations won't work. 'module' object has no attribute 'bindtextdomain' Cannot import: GTK+ No module named gi