Freebase

All of Freebase's data (all entities and relationships in the graph (including full history), as well as blob data such as descriptions and images) are fully accessible through a REST API (Restful).

The core web services allow you to read, write, and search Freebase's Graph data.

The search API lets you do a fuzzily-matched search for a string that occurs in a topic's name, aliases, or description.

The Freebase Terms of Service limit give users a read quota of 100k API calls per day (rolling 24 hour clock) and a write quota of 10k writes per day. Higher quota limits may be granted upon request to Metaweb. Default quotas on the sandbox are 1,000,000 writes a day, but if you have a non-default write quota on the production server, that same quota will be enforced on the sandbox too.
More information: http://wiki.freebase.com/wiki/API