A good CSS reset is a must-have in terms of website design. A CSS reset file sets basic styles so that they will be consistant amongst all the major browsers. For example, most browsers display headings (h1, h2, h3…) at slightly different sizes. A reset file can make them the same size across major browsers, using h3 { font-size: 150%; } or h3 { font-size: 20px; } or something similar.
Using a reset helps a lot with consistent display. Since all the styles have been reset, there are fewer browser inconsistencies that can plague websites. I have found this to be especially true when dealing with headings, lists, and tables. These seem to be the most inconsistent amongst browsers.
Here are several good websites that offer good resets. They offer everything from just the basic resets that cover all major browsers to a complete reset that removes almost every style from almost every browser one can think of.