Wireframe and interaction design software
OverSite for Mac OS XOverSite runs on multiple platforms, but really flies on Mac OSX. And with Apple's Aqua interface, it looks the best on OSX as well, as evidenced by these screenshots:
Easily compose wireframes by adding widgets, shapes, and custom composite.
Create reusable composites, or laid-out groups of widgets and shapes, and place them in your wireframes.
OverSite for Mac OSX:
Disk Image: Open the "OverSite" disk image that appears on your desktop and drag the TimeBox application icon to the location of your choosing, such as the Macintosh HD/Applications folder.