Evernote acquired Skitch some time ago, and I've been using it religiously on my Macs ever since. It is the most simple way I've found for marking up photos, screenshots and maps. And now Skitch is available on the iPad.
Functionally, it is almost identical to the Mac version. Arrows, lines, shapes, text, etc. in different colors... Grab shots of web pages and maps directly from within the app.
Yep. Evernote does it again. Kudos.