WPF, in conjunction with the .NET Framework add-in model, allows you to address a wide variety of scenarios that require host applications to display UIs from add-ins.
Addins (plugins) for WPF can even do UI. I'm not convinced this is necessary to do UIs with plugins, but it's necessary if you want to allow untrusted code to do it.
Solution Factory is a Visual Studio Addin that exports a solution as a Visual Studio multi-project template, so you can provide best practices for specific development project types.
AnkhSVN is a Visual Studio .NET addin for the
Subversion version control system. It allows you to perform the most common
version control operations directly from inside the VS.NET IDE. Not all the functionality
provided by SVN is (yet) supported, but the majority of operations that support the
daily workflow are implemented.
AnkhSVN supports enough of SVN in Visual Studio to get you the source control overlays in your solution explorer, which is all I *really* need. You might want to consider running it along *with* TortoiseSVN
* Indicates file status within the project files tree
* Tracks adding, moving and renaming of file actions
* Dialogs for most actions using TortoiseHG as its backend.* Compatible with VS 2005, VS 2008 and VS 2010 Beta 2.