Data is as fundamental to web applications as the chip in your computer. Just as the chip is the ultimate piece of hardware on which your computer relies, so is data fundamental to map portals like Multimap or MapQuest.
Multimap gets most of its basic map data from Tele Atlas and NAVTEQ. But more specialised sources of data are just as important. For example, Multimap needs to get address data from stores to enable store locator applications.