An instance of a form or an application?

For the application, see the article in Windows Forms Tips and Tricks.

For a form, you could create the form as a singleton but remember to use
show and hide to display or remove the form.

There isn't one because it is difficult to think of any reason why you would
want to do that.

The more usual case is that you want to create the form if it doesn't exist
OR bring an existing one to the front.

Typically I would use a private property on whatever it is that normally
controls the form so that it creates the form on demand so that something
like obj.MyForm.Show() would always work.

It might be that you have a genuine need for the form itself to be a
singleton but I have never come across a case where it was necessary.

Check out this link:

http://www.codeproject.com/vb/net/Simple_Singleton_Forms.asp

It might prove useful.

Regards,

Cerebrus.