So, you may be writing a snazzy new email template and you'd like to send it to yourself to see what it looks like before you send it to a lead. You could do this by creating a test lead with your email address, but it doesn't need to be that hard.
To send a test to yourself, simply navigate to the Email Templates page (under Settings in the sidebar) and click the paper airplane icon to the right-hand side of the email template in question.
This will open a dialog window where you can input an email address to send the template to (note that your email address is inserted automatically).
Click "Send" and check your inbox so see what your template looks like in action.