Adobe Fireworks
I’m officially adopting Fireworks over Photoshop for website prototyping because of one particular feature. Like Flash, with Fireworks you can create a ‘symbol’, let’s say a thumbnail border, and place multiple instances of that thumbnail border on a page. If later I decide to change the color of the border, I can simply edit the master symbol and the change would filter down to all instances. Oh, and you can auto align objects. I’m going guide-free!
Aside, the UI is a little junky, but excusable.
Did you used to use Photoshop? What about smart objects?