As Search Engine Land pointed out, Bing's investment in their web index and API has done a lot to foster competition in search. It's an essential component for us at DuckDuckGo. Leveraging Bing's core search technology has enabled us to invest more in other areas, like privacy.
Steve Ballmer's initial announcement - YouTube (so good!)
Bing turns 10: Why it's been more disruptive than you think - Search Engine Land
Microsoft celebrates 10 years - Paul Thurrott