Over the last couple of weeks i have been asked "What are the differences between between the Developer Edition and the Test Addition of Visual Studio Team Editions"

Or

"What is the difference between Visual Studio Professional and Standard"

Embarrassingly the answer has been here is the link to the Visual Studio 2005 versions of that information and it isn't 'that' different now. 

Brian just pointed we now have that information available  

Visual Studio Team Edition comparison matrix:

http://msdn2.microsoft.com/en-us/vsts2008/products/bb991841.aspx

Pro vs. Standard comparison matrix:

http://msdn2.microsoft.com/en-us/vs2008/products/bb980920.aspx