Applications that submit requests with an API key are allowed 1 000 requests per 24 hour period.
You can increase the above limit to 100 000 requests per 24 hour period by verifying your identity through the APIs console. A credit card is required for verification, by enabling billing in the console. We ask for your credit card purely to validate your identity. Your card will not be charged for use of the Places API.
Yelp: http://www.yelp.com/developers Start with 25,000 calls per day. Contact Yelp for upgrade.
FourSquare: https://developer.foursquare.com/ 5,000 userless requests per hour
OpenStreetMap: http://wiki.openstreetmap.org/wiki/Develop
Google Maps: https://developers.google.com/places/