It depends on what you mean by "design."
If you mean graphic design, because in it you want to create nice graphics for your user interface, then to cut and integrate, then personally I always use Adobe's Fireworks (originally a Macromedia product). This is a vector graphics tool, so I find it easier to use than Photoshop or Illustrator. However, not for free.
If you mean design, like creating user interface elements and experimenting with different ideas, for example. for prototyping, then pen and paper are the best tool! Use sticky notes or a whiteboard to simulate state changes, it is very easy to iterate and prototype with users. There was even a book written. If you need something digital, but low-fi, then Baclsamiq Mockups is a good choice or something that allows you to draw simple shapes (Paint!).
(the preview screwed the hyperlinks, I hope it will be better when it is published ...)
roryf
source share