When comparing Kaleidoscope vs WinMerge, the Slant community recommends Kaleidoscope for most people. In the question“What are the best diff tools for Git?” Kaleidoscope is ranked 5th while WinMerge is ranked 7th. The most important reason people chose Kaleidoscope is:
Kaleidoscope has a beautiful and simple UI. It displays the two files that are being compared side to side, highlighting all the differences in a way that's easy to understand.
Specs
Question Ranking
Pros
Pro Simple UI
Kaleidoscope has a beautiful and simple UI. It displays the two files that are being compared side to side, highlighting all the differences in a way that's easy to understand.
Pro Diff within a line
When working in code, line-by-line diff can still be hard to spot small changes within the line. Kaleidoscope will highlight what has changed.
Pro Supports comparing folders
Pro Image difference viewer
Kaleidoscope supports image diff viewer in addition to git diff. It's able to find even the slightest changes between two versions of the same image.
Pro Lightweight, quick startup
Binary is less than 3 MB, so it starts quickly
Pro Compare folders and files
Can show what files has been changed in a folder, allows comparing files in tabs.
Pro In line comparison
Can show differences within a line.
Pro Filters
Can filter out files for folder comparison, lines for file comparisons, with regular expressions.
Options also allow to ignore whitespace differences, white lines, case change, line-ending changes, etc.
Pro Free & Open source
Winmerge is a free and open source tool.
Pro Good navigation
Keyboard shortcuts (and toolbar buttons) to navigate to next (previous) difference, side panel shows a map of the files with changed lines and allows to jump to a given place.
Pro Good detection of moved lines
Detects when a block of lines has been moved in the file and shows the relation.
Pro File edition
You can quickly copy changed lines (or files in folder comparison) in both directions with keyboard shortcuts. You can edit the files as well, with syntax highlighting of some languages.
Pro Good shell integration
Select two files and compare them. Alternatively, select one file, navigate elsewhere, select the other file to compare.
Also supports drag'n'drop of files / folders from Explorer.
History of past comparisons.
Cons
Con Paid software
Kaleidoscope is not free, costing $69.99.
Con Mac OS only
Kaleidoscope is a Mac OS X only app.
Con Windows only
It's only available for Windows. No Mac or Linux versions available. It is possible to run in Linux via WINE, although a bit unstable.
Con Development is stopped
No update for years; last official release was made in 2013.
Two forks are still in active development as of 2017.
WinMerge2011: https://bitbucket.org/jtuc/winmerge2011/
As of 2015 it was on par with the historical version, and has additional features such as showing only differences and a 64-bit version.
Winmerge-v2-jp: https://bitbucket.org/sdottaka/winmerge-v2 and https://github.com/sdottaka/winmerge-v2
Adds support for 3-way merge, diff-only view, and improvements to the diff algorithm among other additions. Has a 64-bit version.
An 'official' list of forks is maintained (despite the url) at http://www.geocities.co.jp/SiliconValley-SanJose/8165/unofficial_winmerge_nightly_builds.html
Con No 3-way merge
Cannot merge 3 files, can do only comparisons by pairs.
Makes it unsuitable for merging operations, still useful to compare two versions in the history.