You would be surprised but the choie of SaaS search APIs is really small -- even if you are prepared to pay.

Yahoo BOSS (which probably will disappear when Yahoo pulls out of search) and Bing API, basically. I don't believe Ask has an API.

This is a current issue for projects I am working on

Ask does have an API. It's just not open to the public, but is accessible via private agreement.

Because I'm already using BOSS for some things and it will probably become Bing at some point. I'm open to others.