Mockups and prototypes

What do you use to make a rough sketch of how your User Interface should look like?

I like to draw something on a whiteboard in a meeting. When finished I take a digital photo so I can send the design to the participants. Recently I was told to have a look at an online tool called Mockingbird. I played with it for a little bit and I really like it. Any of you using other tools? Pro's and con's?

A sketch on a whiteboard

Something made with Mockingbird