[mahmud]

I use a firefox plugin called "Pencil" to make GUIs and design interactions. For more complex stuff I go back and forth between DIA and Paint.NET (a graphics editor.)

The first is good for designing networks and servers, as you can draw lines between "nodes" and the lines snap into place. The second is good because it has layers. I put each object in a separate layer for maximum ease of use.

[kelnage]

Dia has layers too. See Diagram -> Layers.

[Says the Dia contributor :)]