If someone missed one or more of the emails in an auto-campaign, or you wanted to send an email associated with an auto-campaign to a contact who isn't in the campaign, you definitely can!
Click into the campaign email that you would like to send.
Now, click Quick Send and choose the contact you wish to send it to.
A few notes:
Sending this email to someone adds them to the exclusion list, so if they end up in the campaign, they won't receive it again.
If the person already received this email and you are intentionally sending it again, it will send to them.