Here's an article that reverse engineers Amazon's implementation; http://bjk5.com/post/44698559168/breaking-down-amazons-mega-dropdown http://bjk5.com/post/4469

Here's an article that reverse engineers Amazon's implementation; http://bjk5.com/post/44698559168/breaking-down-amazons-mega-dropdown http://bjk5.com/post/44698559168/breaking-down-amazons-mega-... - not the first implementation by a long shot, as pointed out elsewhere in this thread, but perhaps the first implementation for the web.