No matter what I am doing, I always have Adobe Fireworks open. Even if I don't need it.
For those of you don't know what Fireworks is - it is a program used to design graphics. It is mainly used to design websites and other user interfaces, however many use it to simply prototype their ideas. Think Photoshop - but for anything other than photos, although it does a good job at photo editing too.
Adobe Fireworks allows you to slice your design in to a working interface. It doesn't produce pretty results for the web, but you can export a working webpage. Adobe Dreamweaver will help you take the graphics and create a fully functional website however Adobe Fireworks CS4 will introduce better support for creating websites without the need for Dreamweaver.
If you find Photoshop too complicated, then Fireworks may be your friend as it is far easier to get to grips with. There is a lack of Fireworks tutorials on the internet which may put off some users. But Adobe Fireworks is a very powerful tool. It is both quick and easy to make an idea work, and in the world where new user interfaces and websites crop up all the time - it will keep you ahead of the game.