Urban legends predate the internet, but with the proliferation of e-mail, you're more likely to come across one of those urban legends. You can use the internet to de-bunk or confirm those net-born legends and scare stories.
snopes.com has one of the more comprehensive with several different categories and a search engine. When you find the legend or story you've been looking for, there's a color-coded key that tells you the validity of the story,
truthorfiction.com opens up with some of the latest net-rumors making the rounds. This site also categorizes the legends for you to look up, plus it disects the anatomy of a rumor and how to debunk something that's false.
urbanlegends.com has a discussion board you can sign up on, and the categories are mainly responses to e-mails that had been posted in the group.