Paul Kedrosky mentions Gazerk. which provides search within the (video) game vertical. Although not explicitly so, by serving results in the gaming vertical it effectively becomes the first search engine which provides results from more than one social media corpus (blogs, message boards and reviews).
A search for 'half life' (a very popular PC game) produces 191, 938 results in the games category, 1, 590 in reviews, 1, 636 in reviews, 2, 409 in screenshots, 3, 760 in forums, and 103 in blogs. I'm not yet sure of the precise meaning of all of these categories (especially 'games'), but the aggregation of reviews, blogs and forums is very interesting.
Gazerk is 'powered by Kosmix' which is a portal into a number of different search verticals with a similar look and feel to Gazerk: Health, Video Games, Finance, Travel and US Politics.