It compares and allows you to merge the files.

Beyond Compare is a very intelligent file comparison tool. I've been using Beyond Compare products for years, both at home and at work. As a developer, you will often come across a need to compare two files. There are free tools on the internet that can do this, but Beyond Compare takes it a step further, integrating with the OS, making it super easy to compare two files. It goes beyond that, allowing the user to even compare entire folders at the same time. This is fantastic for comparing a local directory with a backup folder that you might have stored somewhere else, like a USB flash drive. I also love the fact that it is a cross platform application. I have used this product on OSX, Windows, and Linux.

Beyond Compare is top notch for doing any sort of file comparison. If you are doing software development, this is a must have tool. It can show you line by line the differences between lines of code in different versions of source code, and also compare two folder and sub-folder structures to show you where any differences are. Once you find the differences, it allows you to easily merge any given line into the other file with one click. What really sets Beyond Compare apart from other diff programs is the really nice GUI and ease of use. It can also do things like show you differences between two picture files and tags on mp3 files. The Pro version lets you do integration with subversion as well.