Since "guest" access was introduced, you can append the: ?guest=true URL parameter to any content servlet URL, the login page will not be shown if that content is accessable by the guest user (NOTE: if they do NOT have permissions on the content then the login page will be shown as usual!)
This means you can embed Alfresco content URLs in emails etc. using the guest=true parameter to mean that non-Alfresco users can see the items without needeing a real Alfresco user set up.