Qwerly is a people search engine for the social web. The service can be used to connect a user on one social network to accounts on other networks. Search using a Twitter username and Qwerly returns data and other social information links (Facebook, LinkedIn, Foursquare, Plancast, etc.). The API provides a simple, REST API with JSON response to use Qwerly's service within your own applications.
Uberblic is another instance of what may become a trend in the world with thousands of APIs: the Meta-API. The Uberblic Doppleganger API is currently in beta, but it is quite ambitious in that it looks to position itself as a API cross-reference or mutli-plexer. Submit an ID from one API to the Doppleganger, get 4-5 IDs for matching content from other APIs.
As a consumer of APIs, one thing you encounter every day are API rate limits. Just about every API has limits on the number of calls you can make against their API. As developers, we accept the limits because in many cases we are getting the API for free. And in some cases, even the rates aren't enough for a provider to get what it needs, as when Google put the kibosh on Translate. Are limits simply a sign that a provider needs to find a strategy that scales?