JQuery is, in my opinion, the most unique JavaScript libraries in terms of being able to modify content on-the-fly. One such incredibly useful example that I'm able to apply often, is adding PDF icons and _newwindow targets to all URLs with .PDF as an extension on all pages in a site without ever touching the source of the pages themselves, but rather just on one common include page.
Well, this library just got a major injection of Redmond steroids by the announcement, by way of Dion Almaer of Google, that Microsoft will be adding it to its standard development platform. Scott Guthrie has a quick demo of IntelliSense integration with JQuery in the ASP.NET toolset.
It's neat to see technologies such as Hibernate, ORM, and Linq or actual implementations, such a JQuery in this case, span the borders to what I consider the three development realms - Open Source, Java and Microsoft.