You don't say where you are, so it's hard to be authoritative, but the reality that Mapquest, Yahoo Maps, Garmin, Magellan, Google, TomTom, and so on all end up buying map data from a relatively low number of sources. In North America, the two leading vendors are Navteq and TeleAtlas and the both accept corrections from users. As you'd expect, there can be a substantial delay between the time you tell them about a road change (the may require exernal validation, etc.) and the time that propAgates into your favorite mapping product. Here are links for reporting corrections to each. Google Maps actually lets you update street address geocoding information. I don't think it'll let you add new roads, though.