What options exist for ad free web browsing on iOS?

Use Privoxy as an ad-stripping proxy server on a Linux box such as the inexpensive Raspberry Pi featured in this article: http://kimondo.co.uk/raspberry-pi-as-an-adblock-server-for-ipad-iphone-android-and-anything-else-on-your-network/

I have tried this and it works well. (Obviously no jailbreaking is required for this solution).

Try the Diigo Webbrowser. It recently added an Ad Blocking feature that's off by default.

If you have a jailbroken device, you can use the tweak "Adblocker" ($2) to block ads in Mobile Safari. It includes options for pre-defined blacklists and user-defined blacklists. Here's one blog post demonstrating the tweak and another one.