You can access details about email logs from within the administration interface from Admin > Monitoring > Email Logs.
This will not show you the email body however, just the recipient and delivery status.
The Compliance BCC email mentioned by Barney Rubble is the other option to be cc'd on future emails.
You may also find the messages you want in the 'EmailMessage' object which will contain the emails sent from within the UI (Case responses). I'm not sure if other emails sent from the UI will appear there. Emails sent from Workflow etc shouldn't appear in this object.
Other than that I am not aware of there being any other way to retrieve email content of emails sent by the system.