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.
AvalonEdit is the WPF code editor control for SharpDevelop 4 -- I'm thinking about trying to add PowerShell syntax to get it to work as a code editor for PoshConsole...