Back in the 90's application developers often exposed their API through COM Automation. This allowed third-party developers to attach to and manipulate running applications through a rich set of extension points. Unfortunately, that spirit of interoperability has largely fallen by the wayside in the .NET era.
Crack.NET tries to bring back some of that power by opening up WinForm and WPF-based .NET applications. With a rich GUI that puts Visual Studio's property inspector to shame, users can attach to most .NET applications. This relies entirely on the .NET infrastructure, the original application developer doesn't need to provide any explicit hooks.
Once attached, users are free to explore and manipulate the running application. However, the real fun comes in when you start scripting. With IronPython scripts, developers can inject code into running applications to add whatever features they see as missing.
As .NET and the DLR become more popular, we could see a whole new cottage industry for add-ons to applications that were not meant to be extendible.