Q:

How do I send to Multiple Email Addresses for a contact in CiviMail?

Is there a way to configure CiviCRM so that it will send emails to all email addresses on a contact record? 
If not, then how do you best utilize multiple email addresses on a contact (individual, organization or household)?

A:

Yes! Go to /civicrm/admin/setting/preferences/mailing and check "Enable multiple bulk email addresses for a contact". Then check the boxes for all of a contact's addresses that should be included in a bulk mailing.
You can also reach this settings page from Administer menu > CiviMail > CiviMail Component Settings.
It's only available on relatively recent versions of CiviCRM (4.5+ if my memory serves me right)
Note that multiple email addresses for a single contact are still useful, even when the 'Enable multiple bulk email addresses for a contact' box is not checked. In this case, the flow is as follows:
CiviMail will attempt to email the primary email or if no email is set as primary, the first email it encounters. If CiviCRM receives a bounce report from this email, it may decide to put the mail on hold (depending on the type and quantity of bounce reports received). Next time CiviCRM sends an email to this contact, it will ignore the on hold email and attempt to send to the next email address for that contact (if any exists).
Multiple email addresses on one contact are also useful to avoid duplicate contacts, e.g. if a contact sometimes uses a home address and other times a work address.

