Free Maps API

I am making a free PHP based web app and I would like to use Bing Maps but it's not free after a certain limit.

MapQuest seems promising but... it's MapQuest... WTF!

...Google maps? I know they have usage limits too, but they are pretty high. You need to be realistic about expect usage - will you load more than 25,000 maps/day? I doubt it.

http://www.openstreetmap.org/

Basically what @DomainBop said

+1 to OSM