For Delphi for .NET users, even on .NET 1.1 you can add extra properties and functions to a FCL class or classes in other people's assemblies you can not modify. Yes, that language feature is named Class Helper. You can find more information about it by searching on Google.com.
After reading this post from Guy, the author of .NET Internationalization book, I finally knew Microsoft clone Borland/CodeGear again in .NET 3.5. The language feature is nearly the same except different syntax are used.
In all, nice to see Microsoft does something I like.
BTW, this feature is only available in .NET 3.5, while Class Helper has been there for years.