Skitch is an app designed to let you mark up screenshots or photos with comments and shapes, or draw something freehand, and then send your work to others or upload to your social media site. Skitch has some cool features built in, and it installs easily on Mac OS X.
The Skitch interface is like any paint tool, with a set of basic tools to the left and a palette for your artwork on the right. If you're working with screenshots or images, they are displayed in the right pane, too. Putting a comment in a block or arrow is as simple as dragging the shape to the correct location and typing the text. You can use Skitch to highlight areas of a screenshot or photo with color or a circle drawn around the interesting area, or obscure parts using a pixelation function. You need to have an account on the publisher's servers to even use Skitch, which is annoying, and contents of Skitch are saved there. There are size limitations with the free account, so heavy users will need to upgrade to one of several account levels.
The problem we have with Skitch is that everything it does is done better in other apps. The need for an account and to log in every time you launch the app is frustrating, too. In the end, we got so frustrated with Skitch that we uninstalled it. Based on feedback from annoyed users, we aren't the only one.