Paintbrush is a Cocoa-based painting and illustrating program for the Mac, very similar to the Paint application on Windows. Paintbrush is easy to install and can support BMP, PNG, JPEG, and GIF files.
The Paintbrush interface is very simple, with a floating menu with basic tools selectable for drawing on the left, and a menu bar at the top. If you've worked with Paint, Paintbrush looks and behaves the same. It takes only a few seconds to select a tool and draw with it. Paintbrush is all freehand art, although there are the traditional handles for things like circles and rectangles. While not as powerful as third-party illustration tools, Paintbrush can do many basic illustrations. It is also good at importing screenshots and highlighting areas or manipulating the image.
Paintbrush is easy to work with, and if you work on multiple platforms it is handy to have a familiar tool on both Windows and Mac. Having said that, Paintbrush is a basic illustration tool, and as long as that's the way you want to use it, it's great at that task.