AppExchange Bling: DemandBase

I've purchased quite a few lists in my time - and this really is like the "iTunes for buying leads", as DemandBase claims.  You build the profile for the type of customers you want to target, and then drag-and-drop the list of leads into your basket to purchase.  Best of all, DemandBase de-dupes auto-magically, so you don't buy leads you already have (does iTunes do that?).