Evernote, whichin August, has just released an iPad graphics app under the same name. It's a fun and useful graphics product on its own and a decent accessory product for Evernote devotees as well.
An Android version of Skitch was released previously.
Skitch on iOS (download link) is very nicely designed and quite a lot more capable than its simple user interface would lead you to think. While its image modification tools are rudimentary (you can crop an image, that's about it), it has the markup tools most people will need: You can draw lines, arrows, boxes and ellipses, and add text. These are what you need to take an image--a photo or screenshot--and mark it up before you send it on to someone else.
There's no overload of tweaks and options for your markup tools. There's only one type of arrow, for example, and you can select from only eight colors. This is a tool for commenting graphically, not creating art or designs from scratch.
You can get images into Skitch through the iPad's camera, the camera roll, or from Safari or Maps. Or you can start from a blank slate. Once you've marked up your image, it's short work to save it to your Evernote notebook or photo library, or send it on via e-mail or Twitter.
That's all Skitch for iOS does. The key is how smoothly it does it. I haven't seen an app that makes marking up a Google Map, for example, as easy as this. From getting the image to editing it to sending it on, it really could not be easier or go more smoothly.
Evernote CEO Phil Libin told me recently that he saw Evernote as the central app in his company's strategy to reinvent productivity software. Skitch, he said, was also key to this. With the iOS version of the software I can see what he was getting at. After just a short time with this app, I'd say it's a must-have for iPad users. Skitch is free.