Free File Comparison Tools

ExamDiff is a freeware Windows tool for visual-file comparison. It features toggles between the first and second files, automatically detects file changes and prompts the user to compare files again, has a one-push recompare function that attempts to leave the viewer's focus in the same place as before, and saves the standard Unix DIFF file.

A visual text file differencing and merging tool for Win32 platforms. Useful for determing what has changed between project versions, and then merging changes between versions

GNU Diffutils
You can use the diff command to show differences between two files, or each corresponding file in two directories. diff outputs differences between files line by line in any of several formats, selectable by command line options

ComponentSoftware Diff (CSDiff) is a free advanced file difference analysis tool for Windows. Use it to analyze what changes have been made between two revisions of the same file or folder. CSDiff is commonly used to analyze program source files, HTML documents and MS-Word documents.

xxdiff is a graphical browser for viewing the differences between two or three files, or between two directories, and can be used to produce a merged version.

KDiff3 is a graphical text difference analyzer for up to 3 input files, provides character-by-character analysis and a text merge tool with integrated editor. It can also compare and merge directories. Platform-independant.