This directly from OCLC:
Now anyone and everyone can create apps and mash-ups using library data from WorldCat, the world’s largest library catalog. Because the WorldCat Basic API is here!
This new API is a simplified version of the WorldCat Search API. Send it RESTful URI queries in OpenSearch, and it retrieves results in RSS or Atom. The WorldCat Basic API can support up to 1,000/queries per day for noncommercial use. In addition, the WorldCat Basic API provides: