Scott Guthrie showcased the extensibility of Visual Studio 2008 last week at the DevConnections conference in Las Vegas.
For those not in the know, World of Warcraft is the number one Massive Multiplayer Online Game right now for the PC with over 9 million monthly subscribers worldwide (myself being one of them). WoW uses an xml based scripting language called (LUA) that allows users to create in game "Add-Ons" enabling rich features not originally built into the game. There is entire community built around creating these modules with sites such as Curse Gaming, WowUI, and Wow Interface. These modules are their introduction to the world of programming for hundreds of thousands of people each day.
Allowing them to use Visual Studio 2008 Express (free download) removes many roadblocks for creating them and brings the power of the VS debugger/ide into their fingertips. This is an excellent case of the extensibility being built into both Visual Studio 2008 and the .NET Framework 3.5 (which itself has new Add-In Namespaces that are used throughout, including inside WPF - think pluggable UIs. Microsoft is also allow developers to use the Visual Studio IDE shell itself to act as building blocks within their own applications.