The Interop Forms Toolkit allows you to create .NET Forms and User Controls that can be used in VB 6.0 applications. This allows you to migrate VB 6.0 applications to .NET over time (a form or part of a form at a time). However, the toolkit relies on features from the Microsoft.VisualBasic assembly and the VB.NET compiler so it doesn't work with C#.
There are a couple articles/samples on CodeProject.com that discuss the toolkit and how to use it with C#.
Interop Forms Toolkit 2.0 Tutorial
VB6 - C# Interop Form Toolkit
Beth Massi has several articles and webcasts on the use of the Toolkit you can use for reference. Check out her blog for links to resources.