I Thought Wearing White Was Reserved For The Bride At A Wedding

I was invited to six weddings last year, I have one for later this year and my best friend just recently got engaged. She's planning her big day for sometime next year.  I've noticed some things at these weddings that I always thought people weren't supposed to do.

I've always thought the cardinal rule was the bride is the only one who wears white.  However, I've seen guests where white at a few of these weddings.  Did I miss something?  Is that an antiquated rule?

I was really curious about this whole wearing white thing so I jumped on the internet to see what the deal was.  I came across this article on Yahoo! that clarifies that situation along with some others -- like the couple keeps their wedding registry open for a year and whether or not you should bring children to the party.  Oh, and how much you spend on a gift.

Do you know of any other rules one should follow at a wedding?