The process or converting large VB6 apps to .NET is a daunting one.  How do you convert a giant app with dozens of forms?  Do you start a new C# or .NET app and use the business logic from the VB6 DLL?  Do you write new business logic in .NET and access it from VB6?  Not simple questions to answer basically because the migration story is really not great.  Using the conversion wizard does not produce acceptable results so what do you do?  Stop all new development and start converting for the next 12 months?
Sylvain, an architect where I’m currently on contract, pointed me to a cool new power pack tool called the Interop Forms Toolkit 1.0 available for free from Microsoft’s Website.
http://msdn2.microsoft.com/en-us/vbasic/aa701259.aspx

The toolkit’s goal is to help you migrate VB6 apps slowly but surely by being able to call VB.NET Windows Forms directly from VB6 like they were regular VB6 forms.   From the point of view of the VB6 dev, the .NET form feels like a regular VB6 forms, wicked!  However, out of the box, it works only with VB.NET but Leon Langleyben created a cool project on The Code Project Website to solve this issue.  Now you can call C# Windows Forms from VB6 using Leon’s mod.
http://www.codeproject.com/useritems/VB6_-_C__Interop_Form.asp

See the toolkit in action in this screencast:
http://blogs.msdn.com/vbteam/archive/2006/09/23/768719.aspx

I can’t stress how fantastic this toolkit is because it solves a five years old problem: VB6 migration to .NET that make sense.  The toolkit was developed by Jon Rauschenberger’s team at Clarity Consulting.  Jon is a Regional Director based in Chicago.  He was interviewed recently on .NET Rocks about the toolkit:
http://www.dotnetrocks.com/default.aspx?showNum=184