Fireworks is often overlooked. Many (most) web designers today use Photoshop to design their websites, and a lot of them have never considered or (*gasp) even heard of Fireworks. Adobe Fireworks is made for creating websites. That is what it was built for, and it does it well. Fireworks enables you to rapidly prototype and design for the web. With it you can create a clickable PDF that will act as a prototype. Also, you can quickly create export and optimize slices (Fireworks is better at optimizing images than Photoshop).
Now don’t get me wrong, Photoshop is an extremely powerful tool and is amazing; it has way more features than Fireworks, but if all you are doing is designing websites, then you should use software that was specifically made for doing just that. Another excellent feature to note about Fireworks is that it is compatible with Photoshop. For the most part, you can open a psd in Fireworks, and it will recognize all of your layers and styles, and with CS4, compatability between Adobe’s programs have been further improved.
If you are a web designer and have not looked into using Fireworks, you should at least try it out and see for yourself how efficient it really is. There are hundreds of other excellent features Fireworks has to offer, read more about them at Adobe’s site.