Merchant Circle just launched an API to allow development of applications on top of their content. The site is loaded with unique business descriptions, reviews, coupons, SMB blog posts and other fun data that they think will be useful, particularly to the Foursquare’s. GoWalla’s and Loopt’s of the local world.
I believe 2010 will see a flood of local search APIs from established players such Yellow Pages Publishers and Data aggregators which means we will see a lot of new interesting location-based apps.
Here’s my current list of Local Search APIs.