With all the great changes to the New Project and New Item dialogs you may be perfectly happy with the list of things that are presented. But let's say, for argument sake, you want to get rid of some of the entries. I'll use the New Project dialog (CTRL + SHIFT + N) for these examples but this can also be done with new items as well.
Assuming we want to get rid of the C# Windows Forms Control Library:
Go to C:\Program Files\Microsoft Visual Studio <version>\Common7\IDE\<Project or Item>Templates\<language>\<project category>. You will probably have to dig deeper depending on what you are after and the path may be slightly different based on Visual Studio version. In this case the actual full path on my machine to the Windows Forms Control Library zip file is "C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\ProjectTemplates\CSharp\Windows":

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