What options exist for ad free web browsing on iOS? In OS X I can use extensions in most browsers to block ads. This offers an distraction free browsing experience.

For example: In Safari, Chrome and Firefox I can use Adblock or Adblock Plus. Both extensions use filter lists which you can find here. You can also manually add filters and add sites on which you accept to see ads (e.g. apple.stackexchange.com).

What options exists for ad free web browsing in iOS?
Try the Diigo Webbrowser. It recently added an Ad Blocking feature that's off by default.