Some people make it a habit of adding everyone who mails them to their address book, something that happens automatically in services such as Gmail. In Mac OS X Mail this is possible thorugh a simple keyboard shortcut.

When reading an email, press Command-Y to add the sender to your address book straight away. Mail will even add the persons name, it it appears in the From field. To add more details like a picture or address you will need to actually open up address book sepereately though.