The Google Custom Search .NET library by Google interacts with the search and customization functionalities of the API. It contains guides to setup with authentication and authorization, simple API example, web applications, and service accounts. Besides this .NET library can access custom search, additional APIs can be also supported with their respective documentation. A complete list: https://developers.google.com/api-client-library/dotnet/apis/
Microsoft continues to built out its cogitative services API portfolio with two more APIs now generally available. The Bing Custom Search API and v7 of the Bing Search API are both GA and ready for production use. The Customer Search API enables targeted searches, and v7 adds filtering.
With Twitter reducing its search history to four days, it is a challenge for applications that are dependent on search results, that went not days but even months back. Several applications have taken on the challenge to fill this void and are building search indexes on top of Twitter. Topsy is one of them and it's sharing its results as an API.