ECMerge is a comparison and synchronization tool with two and three-way merge for text and folders and difference on binary, text, images and folders, locally, via FTP/S, SFTP or in version controls. For Windows, Mac OS X, Solaris and Linux.
Guiffy is a diff merge tool and folder compare application and API. SureMerge is the most trustworthy source code 3-way merge tool available. Advanced features include tuneable inline diff highlighting, syntax highlighting and a blend of 3 diff algorithms.
Difftastic is a diff tool that compares files based on their syntax. It supports over 20 programming languages and when it works, it's fantastic.
Text Compare lets you instantly see the difference between two pieces of text. Compare text by paragraph, word or character. Paste in two pieces of text, and Text Compare will highlight what has been inserted in, or deleted from, the original text.