PCmag.com has detailed instructions on how to delete accounts from these websites.
Social Networks
Classmates.com, Facebook, Friendster, LinkedIn, MySpace, MyLife.com
Online Retailers
Amazon.com, Audible.com, Blockbuster Online, eBay, iTunes, Netflix, PayPal
Blog Services
Blogger, Twitter
Sharing Services
Flickr, Photobucket, YouTube
Online Services
AOL/AIM, Apple's MobileMe, Google, Windows Live ID, Yahoo