Rita is a paint program for MacOS X that attempts to be limitless, flexible and easy to use. Unlike traditional paint programs Rita does not restrict you to a particular image size or resolution. Instead, it uses an infinite canvas. You can make the image as big as you like and zoom in as much as you want without ever encountering any big blocky pixels.
Rita is distributed as fully functional shareware -- no time limit, no nag screens. Rita is most fun when using a Wacom drawing board or a similar device.