Apple Preview allows you to sign PDF documents with your FaceTime Camera using Mac OS X Lion or Mac OS X Mountain Lion.
- click the button for the edit toolbar
- click the signature button
- click “Create signature…”
- write your signature on a plain piece of paper and hold it up to the camera
- click Accept
- click in your PDF document to insert
- resize and position your newly inserted signature
Once saved, you can reuse your signature any time.