Encrypt wisely points out that:
"one of the most easily overlooked steps towards defending your digital privacy is preventing old skeletons from your past being unearthed. Most internet users have had many different eMail addresses over the years, and have abandoned accounts on facebook, livejournal, help forums or other communities. These represent a threat to you if you don't log back in to delete the account; somebody intent on stealing your identity could easily get into an old account and find information that'd you'd forgotten it contained..."What follows is my edited version of their list of sites where people most commonly have dormant accounts, and where possible, a link to where you can delete that account: (Encrypt encourages the sharing of their tips.)
AIM/AOL: Notoriously difficult, best bet is to phone 888-265-8008 - they will ask your security question
Amazon: Login > Contact Us > Select "Close Account" from dropdown reasons, OR you can phone 1-866-216-1072
Classmates.com: Call 1-425-917-5000, hit 1, then hit 2, then ask the representative to find and cancel your account.
eBay: http://cgi1.ebay.com/ws/eBayISAPI.dll?CloseAccount&guest=1, note that eBay retains all your customer information indefinitely even after you delete your account.
Facebook: http://www.facebook.com/help/contact.php?show_form=delete_account - note that "deactivate your account" is NOT the same thing; you want to delete, not deactivate.
Friendster: Login > Settings > Cancel My Account
Google: Log in, go to https://www.google.com/accounts/EditServices, it will let you delete specific service accounts, or close your account across all services
Hotmail/MSN: http://mail.live.com/mail/CloseAccountConfirmation.aspx, takes 270 days after confirmation to be deleted
iTunes: View My Account > Edit Payment Information > Remove Credit Card, and then go to http://www.apple.com/support/itunes/store/browser/ to contact and request account deletion.
LinkedIn: Call 1-650-687-3600
MySpace: My Account > Account > Scroll down to bottom > Cancel Account
Ning: Requires you to contact them through http://help.ning.com/?page_id=23
PayPal: Profile > Account Information > Close Account
Twitter: settings > Delete My Account
YouTube: Login > My Account > Account Settings > Delete Account