[nolok]

Balsamiq

[pbreit]

I like balsamiq specifically because it is rough. That eliminates a lot of the wasted time arguing about pixels.

[burnblue]

I found Balsamiq completely unusable for large projects because of the lack of master pages. Maybe they've addressed this by now; that was ages ago.

[logicalmike]

I'll second this. I've been a paid user for years now.