My UI/UX design company developed our own tool to replace Photoshop for our rapid prototyping and UI design work. It is publicly available by now. http://www.antetype.com
We switched from Photoshop to Antetype company-wide now and we don't miss it (It's still here for Icon Design though). We also felt Antetype superior to Fireworks as it has ways do deal with dynamic layouts and prototypic interactivity.