Ad-blocking apps have surged to the top of Apple's paid iPhone app chart in the US, just days after the release of iOS 9. As of Friday morning, ad-blocking apps accounted for three of the top five paid apps, and four of the top 20. Peace, which launched yesterday at a price of $2.99, is currently at the top spot, followed by Crystal ($0.99) at number two and Purify Blocker ($3.99) at number four.

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There's been a lot of recent debate around the ethics of ad-blocking, following the release of the WebKit Content Blocker API in iOS 9, which allows for developers to block ads in Apple's Safari mobile browser. Proponents of the software frame the issue as a matter of consumer choice, while others say the spread of ad-blocking could threaten the revenue model for most news sites and online publishers. Others have noted the irony of paying to block ads (which effectively pay for free content), though as Apple's recent charts show, it's a price many are willing to pay.