Typically if your going to a design position bring your portfolio, (print and web) and show that to them. If your going in as a developer show any application you have worked on (or where they were if there aren't still running).
Understand the interview may be skewed out of your favor. For example I've interviewed places where the position was titled "Web Designer" but while talking with the manager/owner it was more like "Senior-.NET Developer" who just wanted a developer that knew CSS. It's important to know when to show-off design-work or put that aside.
A few questions I've asked at interviews before would be like:
What's the W3C?
Can you create a CSS based-layouts?
What tools do you use to create sites, (Outside: Visual Studio, Dreamweaver, Photoshop)?
Which is better quality JPG or PNG?
Is HTML a programming language?
Have you worked with XML/XSL/XSLT?
Any Mac or Linux experience?
What programming languages do you know?
There's a lot more questions I could ask but it's enough to prep for. :)
Hope this helps!