This app is one that actually costs something. I usually avoid those, but I was so impressed by Explain Everything that I coughed up the $2.99
Explain Everything allows you to create, narrate, and mark up screencasts and slideshows. To see a video that I created with this app, click here and watch the library intro video.
I love this app because the audio and markups are tied to the "slide" that you're viewing. If I mess up when I'm talking or circling a link (anyone who's sat through a class with me teaching knows that this happens pretty often!), I can easily toss out that slide and re-record just that part without having to worry about scrubbers and such.
It's not as robust as ScreenFlow or some other options, but I feel like it's a sweet spot at the intersection of a good end product and being easy to use.
Visit the Explain Everything website or find it in the App Store.
And don't forget to download the Reeves Library app!