"I don't know why I did it, but I decided to translate the Perl module Algorithm::Diff into C#, since there aren't any C# libraries yet for finding the differences between two lists."
The DiffPlex project is a combination of a .NET Diffing Library with a Silverlight and HTML diff viewer. It is released open source under the MS-PL license.
A nice commecrial XML editor that not only has excellent validation, xslt and xpath and xquery, andformatting objects (fo) features, but supports an XML data grid view and XML Diff & Merge! It even supports generating SVG graphics via XSL transforms on data files, remote editing over FTP or WebDav, and is available as an Eclipse plugin. Only $48 for Academic use.