Earlier this summer, Facebook released a feature that automatically opts you in and recommends that your friends tag you in photos when it recognizes your face. Here's how to opt-out of it.
Start by clicking the drop-down menu on the top-right of your screen, then select "Privacy Settings." Choose "Edit Settings" next to "How Tags Work," then click the link next to "Tag Suggestions" in order to disable it.
Do note, however, that this setting does not prevent friends from tagging you entirely—they can still do so manually.