Following to earlier post by Nick about Visual Studio 2008 SKUs I just want to say that extensibility isn’t supported by Express editions of Visual Studio but you can develop and use extensions in all commercial editions.

Moreover, Domain-Specific Languages tools are supported by Professional edition or above so you can develop a DSL with Professional edition or above but can deploy a DSL to all commercial editions including the Standard edition.

Generally Professional edition is enough for someone who wants to develop and use extensibility options but above editions can be better because of the features that they offer for debugging, testing and other general-programming tasks.