Illustrator is awesome. Also, pencil and a piece of paper. Sometimes I create physical models of complex UI objects if I need to grasp their animations/behavior

Illustrator is awesome. Also, pencil and a piece of paper. Sometimes I create physical models of complex UI objects if I need to grasp their animations/behavior. Some designers I knew liked Balsamiq, or Blend, but everyone knows how to draw on paper. It would be harder to design in Photoshop, methinks, since when designing you're dealing with abstract shapes and lines (vector) not raw pixels (raster). I mean sometimes you do :) I created and prototyped this cool video-preview control entirely in Paint.NET, literally changing pixels to create gradients.