The Microsoft Live Search Web Service is an easy to use SOAP service. These are a few key highlights:
MSN Search Web Service only has one operation, which is named Search. A cached version of the page is available, as part of the Search result.
MSN Search has a search request limit of 20,000 requests per day.
There are 250 maximum results returned with each search.
The home page for MSN Search Web Service is at http://msdn.microsoft.com/live/msnsearch/default.aspx. Extensive documentation, sample code (in.NET), and developer forums are available. You will also need a license key, which they call an AppID, for Live Search. You can get that at http://search.msn.com/developer.