http://developer.ebay.com/windows/Ebay Windows Develope Center.
Can interface nicely with .NET. Needs (free) signup. API is free for a number of limited API calls per month (after that it will just deny the calls).
To get started, look at the checklist on the righthand side of the page.
Indian_Mayhem - as a panel developer, I can tell you that just posting the list of API sites will not help developers. You might want to add information about what to do with that API. For example, I have no idea what is your idea regarding the Speech SDK.