I agree with the others, don't burden yourself with anything except maybe a small notepad.  Maybe a book to read because you might be left sitting around waiting.  Just show up, be confident, answer questions honestly.  You might want to bring a cheat sheet of questions to ask them about what the job will be like, if that sort of thing helps you.  Remember, you're interviewing them as much as they're interviewing you.  Worry more about meeting and getting along with the people then being grilled about technical questions.

I disagree about code.  Don't *bring* code, but make sure they already have a sample of your code on some real project.  Make sure it's code you're allowed to show people, so no code from a previous job.  Open Source code from a real project is ideal.  Realistic code samples answer a lot of questions about what sort of programmer you're going to be, and it shows you're not just a 9-to-5 programmer.  That you're going to expand your skills outside of work.

You should spend most of your time preparing by investigating the company, what they do and how they do it.  Bone up on whatever it is they do so you're not completely blind sided, even if it's just reading a few Wikipedia articles.  This will also let you show honest interest and ask more questions, which is good.

Finally, get a good night's sleep, eat a good meal before hand, err on the side of being early, go to the bathroom before heading in, and turn off your cell phone.