I recently opened a Gmail account and discovered I no longer wanted to use that service. These are the steps I used to cancel my Gmail account.

• Click Settings at the top of any Gmail page.

• Click the Accounts tab.

• Click Google Account settings.

• Click Edit next to My products.

• Click Delete Gmail Service.

• Place a checkmark in the checkbox next to Yes.
• Modify New email address if necessary.
• Press Remove Gmail button.

Inspiration: http://digg.com/