I always struggle when I need to “design” something, because I’m really not that much of a designer. But I really care about the looks of my application. One of the more difficult tasks for me is finding nice colors for a layout. So I was really happy when I discovered Color Scheme Designer and Kuler. Kuler is large collection of color schemes composed of 5 colors, which can nicely be used together in a layout. Color Scheme Designer lets you create your own color scheme from a color of your choice (with complementing colors, etc.)
Let me know if you know of other tools which make your designer life easier!