Skitch has long been a beloved program on the Mac, gaining followers in its early beta days and even more converts after Evernote acquired the app in 2011. Skitch's free iOS counterpart is just as lovely, offering mobile and tablet users a quick way to draw all over PDFs, websites, maps, and photos before sharing them with the world.
It's an app I've found myself using more and more of lately, especially when testing apps or web projects. Whether your collaborators are thousands of miles away or simply unable to meet for coffee, Skitch lets you reach out in a more tangible way than the average email list of notes.
As I mentioned above, when you open Skitch, you have a variety of ways to mark up objects: you can draw on a blank page, take a photo (with a pre-built annotation arrow or box) or pull one from your library, annotate a PDF or webpage, or add information to a map page.
Skitch's rotational carousel makes all of these options simple to select and get started; once you've selected what you wish to mark up, you have a variety of tools at your disposal. My favorite when working with app screenshots is the insta-blur--just draw a rectangle around an area of the screen to instantly mask its contents. You can also add commentary arrows--little checkmarks, Xes, exclamation points, question marks, and even smiley faces that you can tag atop your document.
In addition to these, Skitch's stalwart tools include shape tools (rectangle, round rectangle, circle, and line), text, and standard arrow markup. All of these can be resized and colored after their initial placement, so if your arrow isn't quite the angle you want, you can readjust.
You can share your Skitch creations with just about any service thanks to the app's support for Open In (and, come iOS 8, extensions), though there are already a bunch of defaults built in, including Evernote support. You can also just save it directly to your camera roll for future use.
All in all, Skitch is an excellent little tool for your iOS utility toolbox--whether you mark up lots of documents or just want to doodle a little. It's absolutely worth picking up.