F-Secure AdBlocker is a free ad blocker for iOS.
The app works by using the Content Block extension introduced in iOS 9. It then uses F-Secure's database to block unwanted ads in Safari (not apps), saving bandwidth and speeding up your browsing, while not affecting anything else.
There are none of the sneaky catches you get with some ad blockers. It doesn't require you to sign up for something else; it doesn't sell your personal information; and as F-Secure points out, it doesn't tell some central server where you're surfing or which browsers are blocked. It just blocks local ads, and that's it.
A super-simple iOS ad blocker. Effectiveness is hard to judge and the app store reviews are mixed, but the app is 4-star rated so it must be doing something right.