Bringing along code (of some form) that you consider "good code" and then being able to explain clearly why you think it is good, is one of the best ways to prove and demonstrate that you are passionate about your work and are willing to stand by it.
This may be code you worked on yourself or could actually be open souce code for example that you happen to admire for certain aspects.
Of course many companies will not specifically request code at an interview, that doesn't mean you can't have it with you anyway. Some interviewers might not even think to ask, but would be interested in seeing it anyway. Offer it, but just don't force it on the interviewer if there is little interest.
As Joel and Jeff have spoken about on the podcast/blogs, "Why would you hire someone to be a programmer and not want to check that they can actually write decent code?".
In fact, the way in which the interviewers respond to your code, their level of enthusiasm and the depth of the questions they ask you about it, are very insightful in helping you to decide if you this company is for you.
Sadly I've never been asked to present/discuss some code in an interview. The nearest I've come to is a basic (very) coding test.
However, from what I've seen, software companies should ask for code at interviews more often, software quality could only improve by at least eliminating people who "would be better in another career".