From time to time you download an app that only proofs its worth after a while. When iOS 8 was first released I downloaded every app that I could find that integrated one of the new features. Widgets, extensions, document pickers, you name it. I ended up with a Notification Center with a dozen widgets and a share sheet that kept on sidescrolling for miles.
I've trimmed down my collection considerable since then, but there's one extension that I use more often than I thought I would: View Source.
It's a Safari extension that replicates the Web Inspector of Safari on the Mac. It shows a site's source code, css style sheets and an overview of all media files on a site.
You can view a website's raw html, do a search for a specific div, export the code to other apps (like a text editor), or generate a list of all media assets on a website, handy if you want to batch download a bunch of photos on a page via e.g. WorkFlow.