WinMerge - WinMerge is an open source differencing and merging tool for Windows.
WinDiff - Download Windiff : free files and folders comparison tool
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
AptDiff - AptDiff is a visual text / binary files comparison and merging utility for Windows.
Beyond Compare - Beyond Compare allows you to compare files and folders.
Online Text Compare - Online Text Compare tool allows you to compare two documents in different formats.