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Pro Supports editing files directly
In addition to comparing two files it also allows you to edit them right in place. What's more, the diffs are updated automatically.
Pro Comparing two or more different folders is supported
Meld allows users to compare two or three different folders for differences. But if a user wants to 'zoom in' and compare files contained in these folders, Meld gives you the ability to do so and launch file comparisons between files contained in different folders or in the same folder.
Pro Free and open source
Meld is freely available on Linux, Windows and OSX (through MacPorts, Fink or Brew).
It's also open source and distributed under the GNU General Public License (GPL).
Pro Supports some simple version control actions
Meld supports the major version control systems (Git, Mercurial, Subversion and Bazaar). You can launch file comparisons between different versions to see what parts changed before commiting.
Simple version control actions are also supported and possible. For example: commit/update/add/remove/delete files.
Pro Three way comparisons
You can compare up to three different files for differences. Plus you can edit files from the comparison view and the diff will automatically update.
Through the GNOME Translation project and the translators that have worked for it, Meld is available in multiple languages. You can check if your language is supported in the translation statistics page.
If you can't find your language or if your language translation is unfinished and you want to help, you can do so by joining the GNOME Translation Project.
Con Slow on large files
While larger files are supported, they can take a long time to load. (20,000 lines+)
Con Python 3 not supported
May or may not be an issue, but if you are trying to standardize on Python 3 for all new projects, having to keep 2.7 around just to support a merge tool might not be ideal.
Con No official OSX Support
Some attempts have been made to port, but nothing easy or fully working.
Con Missing contextual command line options
Not all contextual elements are exposed to the cli. Specifically the ability to follow our not follow symlinks, but also things like file system metadata parsing. These options and others can only be toggled via the gui's settings window.