Mailinator(tm) provides users with free, disposable email addresses. Accounts are created when email arrives for them (i.e. there is no sign-up process at any time in using the service).
Mailinator accounts are useful when a user doesn't wish to use their permanent email address on the internet, but still wants to receive email from some source (i.e. web sign-up, software download, etc).
Per October 2008, Mailinator servers received 18million emails per day.