• WinMerge

From author's site:

WinMerge is an Open Source differencing and merging tool for Windows. WinMerge can compare both folders and files, presenting differences in a visual text format that is easy to understand and handle.

WinMerge is highly useful for determining what has changed between project versions, and then merging changes between versions. WinMerge can be used as an external differencing/merging tool or as a standalone application.

General

• Supports Microsoft Windows 98/2000/XP/2003/Vista/2008
• Handles Windows, Unix and Mac text file formats
• Unicode support
• Tabbed interface

File Compare

• Visual differencing and merging of text files
• Flexible editor with syntax highlighting, line numbers and word-wrap
• Highlights differences inside lines
• Difference pane shows current difference in two vertical panes
• Location pane shows map of files compared
• Moved lines detection

Folder Compare

• Regular Expression based file filters allow excluding and including items
• Fast compare using file sizes and dates
• Compares one folder or includes all subfolders
• Can show folder compare results in a tree-style view

Version Control

• Creates patch files (Normal-, Context- and Unified formats)
• Resolve conflict files
• Rudimentary Visual SourceSafe and Rational ClearCase integration

Other

• Shell Integration (supports 64-bit Windows versions)
• Archive file support using 7-Zip
• Plugin support
• Localizable interface
• Online manual and installed HTML Help manual