Guide: The links below are colour-coded to indicate the difficulty level of account deletion.

EasyMediumHardImpossible
Amazon

difficulty: hard

To close your account, contact Amazon by email (via this contact form) and request that your account be closed.
show info...
AOL / Instant Messenger

difficulty: easy

No Info Available

bitly

difficulty: impossible

The best you can do is archive all of your links. Go to the link you want to archive and click the 'x' in the upper right of the shortlink to archive the link. You can also make all of your shortlinks private.
show info...
Blogger

difficulty: impossible

You can't delete your Blogger Account without deleting your entire Google Account. But you can delete your blog.
show info...
Delicious

difficulty: easy

No Info Available

Digg

difficulty: hard

Contact Digg’s customer support and request for your account to be closed.
show info...
Disqus

difficulty: easy

No Info Available

Dropbox

difficulty: easy

No Info Available

eBay

difficulty: easy

A few survey questions will be asked prior to account deletion.
show info...
Facebook

difficulty: medium

show info...
Flickr

difficulty: easy

No Info Available

Foursquare

difficulty: easy

No Info Available

Freelancer

difficulty: hard

Make sure your account balance is positive, then issue a support ticket requesting to close your account.
show info...
GoDaddy

difficulty: impossible

GoDaddy Accounts are apparently retained “to comply with [their] legal obligations” though you are able to clear out most of your information by editing your profile.
show info...
Google

difficulty: easy

No Info Available

Hi5

difficulty: easy

No Info Available

Hotmail

difficulty: easy

No Info Available

ICQ

difficulty: impossible

ICQ is now working on providing a delete feature which will be available in upcoming ICQ versions. Currently, there’s no such functionality.
show info...
ImageShack

difficulty: easy

No Info Available

IMDb

difficulty: easy

No Info Available

Instagram

difficulty: easy

No Info Available

iTunes / Apple ID

difficulty: hard

Contact customer services. You will lose access to any media you haven’t downloaded to your computer.
show info...
Justin.tv

difficulty: easy

No Info Available

LinkedIn

difficulty: medium

There are reports that LinkedIn continues to email people with a closed account. You may need to contact customer services to delete account instead of just closing it.
show info...
MailChimp

difficulty: medium

Account Settings → Account Settings Drop-down → Close my account → Type DELETE and press Delete Account button.
show info...
MediaFire

difficulty: easy

No Info Available

Microsoft Office 365

difficulty: easy

No Info Available

MySpace

difficulty: easy

No Info Available

Netflix

difficulty: impossible

Contact customer services. Even then they may not delete your account under the premise that you might want to rejoin and keep your history and recommendations.
show info...
New York Times

difficulty: hard

Use the form to write to customer services and ask them to close your account.
show info...
Orkut

difficulty: easy

No Info Available

Outlook

difficulty: easy

No Info Available

Pinterest

difficulty: impossible

Accounts can be deactivated, which means that your pins and profile are hidden but not deleted.
show info...
Shopify

difficulty: easy

Select 'Please cancel my account'. Choose a reason then select 'close my shopify store'.
show info...
Skype

difficulty: hard

Contact customer services. You’ll need to know 5 contacts from your contacts list, the month you created your account, and your signup email address.
show info...
Soundcloud

difficulty: easy

No Info Available

Spotify

difficulty: hard

Contact Spotify’s customer services through their contact form and request for your account to be closed.
show info...
Starbucks

difficulty: impossible

They will not delete your account but upon request they can “scramble all of your information so that you don’t receive emails and none of your information is available to [them] for potential fraud”.
show info...
StumbleUpon

difficulty: easy

You can reactivate within 14 days. After that the account is deleted.
show info...
TeamViewer

difficulty: easy

Edit profile (menu item at the top right corner of the page) → Delete account
show info...
Tumblr

difficulty: easy

No Info Available

Twitter

difficulty: easy

No Info Available

Ustream

difficulty: easy

Scroll down to the bottom of the 'Account Settings' page and click 'Delete Account'
show info...
Vimeo

difficulty: easy

No Info Available

WhatsApp

difficulty: medium

From the app: Settings → Account → Delete your account.
show info...
Wikipedia

difficulty: impossible

'A username cannot be deleted.' However, they do have some suggestions.
show info...
WordPress.com

difficulty: impossible

“WordPress.com accounts cannot be deleted.” The best you can do is remove any identifying data from your account.
show info...
Yahoo!

difficulty: easy

No Info Available

Yelp

difficulty: easy

No Info Available

YouTube

difficulty: impossible

You can't delete your YouTube account without deleting your entire Google Account. But you can delete your channel.
show info...