Here are pure open source based web map and geo location database resources. Google map or Bing map is wonderful, but they require business contract for commercial use. It is said that Foursquare recently switched their map tool from Google map to open source based Leaflet API which looks like the picture below. This map tools is quite great and recommendable.
– web API for map : http://leafletjs.com/
Good place to start. Two resources below are not really used for developers, cause those are already included in leaflet javascript API.
– Map style : http://cloudmade.com/
– Map data : http://www.openstreetmap.org/
– Geo location based on ip address : http://dev.maxmind.com/geoip/geolite
Can download free ip address & city mapping file, which can be used for custom mapping code.