It's been mentioned once, but I think the Minds from Ian M. Banks' Culture series is a quintessential example of benevolent A.I.
To maintain a post-scarcity society, humans have turned over control of to the Minds, "hyperintelligent machines originally built by biological species which have evolved, redesigned themselves, and become many times more intelligent than their original creators."
Minds manage everything from resource allocation to war planning. They also control all of the technical, day-to-day operations. Still, "the essentially benevolent intentions of Minds towards other Culture citizens is never in question. More than any other beings in the Culture, Minds are the ones faced with interesting ethical dilemmas."