Because I simply cannot remember which feature is in which edition of Windows Vista, I searched the Web to come up with a couple of useful feature comparison matrixes to back up my memory:

• The Editions (a very basic overview)
• IIS 7 Features and Vista Editions (useful for the Web guys like me)
• Windows Vista Review, Part 2: Understanding the Vista Product Editions (Paul has a couple of useful tables)
• The Windows Vista Secret Decoder Ring (kind of quick reference table)
• Windows Vista editions comparison matrix (Windows Anytime Upgrade screenshot deluxe)

Update And of course the Windows Vista Product Guide.