Beyond Compare

From my answer to this question - two days ago HINT: you can search our site for questions similar to the one that you plan to ask.

I don’t think that you will get a better open source application than WinMerge (although it would do no harm to look at Kdiff3.

Neither of them will handle your int i; for (i ...) -> for (int i ...) example, but they are open source, so you might give it a try, or request the feature from the author.

General
• Supports Microsoft Windows XP or newer
• Handles Windows, Unix and Mac text file formats
• Unicode support
• Tabbed interface

File Compare
• 3-way File Comparison New!
• 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
• 3-way Folder Comparison Work in progress

Image Compare New!
• Support many types of images
• Can highlight the differences with blocks
• Overlaying of the pictures is possible
Version Control
• Creates patch files (Normal-, Context- and Unified formats)
• Resolve conflict files

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