DiffMerge is a useful tool for comparing 2 of the same text files, and merging the differences if you want. It is great for keeping track of the changes between different versions of configuration files, html files, and even revisions in documents. Diffmerge integrates with the Windows explorer, which allows for easy right clicking and merging files within the Windows explorer. It is highly configurable allowing rulesets and options to provide customized appearance and behavior. Diffmerge is also available in 42 different languages and has the same features across multiple platforms including Windows, Mac OSX, and Linux.

It essentially has 3 modes; diff, merge, and folder diff.

Diff - Graphically shows the changes between two files. Includes intra-line highlighting and full support for editing.

Merge - Graphically shows the changes between 3 files. Allows automatic merging (when safe to do so) and full control over editing the resulting file.

Folder Diff - Performs a side-by-side comparison of 2 folders, showing which files are only present in one file or the other, as well as file pairs which are identical or different.