Though not as comprehensive as a 7 style Stout tasting (I should do this again before winter is over), Andy Crouch talks about Stouts, including a little bit about their history and some of the more common variants to the style.
Let me add a newer variant to the list. Belgian Stouts. Most seem to be Belgian yeast applied to Foreign Export Stout recipes.
De Dolle Extra Export Stout
Jolly Pumpkin Madrugada Obscura (sour! let's get crazy)