February 16, 2012
Mac OS X 10.7, aka Lion, adds an annotation feature to the Preview app which can be used to "digitally" sign documents. Instead of
Launch the Preview app, and from the Preview menu select "Preferences". Click on "Signatures" and then "Create Signature". Write your name on a piece of white paper and hold it up to the camera in your computer, align it on the blue line in the "Signature preview" pane, and click "Accept" to save it.
You can have multiple signatures saved, for example I have one where I've printed my name, and one where I've signed it.
Open the PDF you need to sign. (Don't forget that you can convert just about anything to a PDF using the Print dialog and "Save as PDF...".) Click on the Annotations button (a pencil icon) and then the Signatures button (looks like a capital letter S). If you have multiple signature saved use the drop down triangle next to the Signatures button to select the one you want. Use the crosshair to draw a box where you want the signature positioned. Once the signature is where you want it, save the PDF and it's ready to be used.