Requirements:
- free, preferably open-source
- implemented in one of the .NET managed langs
Google found these:
- A Generic, Reusable Diff
Algorithm on codeproject - An O(ND) Difference Algorithm for C#
- Diff/Merge/Patch Library for C#/.NET by Joshua Tauberer
EDIT:
No apps please, only libraries.
You can grab the COM component that uses Google’s Diff/Patch/Match. It works from .NET.
Update, 2010 Oct 17: The Google Diff/Patch/Merge code has been ported to C#. The COM component still works, but if you’re coming from .NET, you’ll wanna use the .NET port directly.