OS X Lion has made signing PDFs easier than ever before. It's been possible to scan in your handwritten signature and sign documents in earlier versions of Mac OS X, but it was a complex process and one most people probably never trifled with. More often than not, I found it easier to simply print out the document, sign it the normal way, and scan the whole document back into Preview using my flatbed scanner.
Lion's version of Preview comes with a built-in signature scanner that makes signing documents far simpler. In the Annotations toolbar you now have an option to create a signature from your Mac's built-in iSight camera. All you need to do is use black ink to sign a piece of white paper, align your signature toward the camera using the onscreen guides, and take a snapshot of the signature. (I haven't used my real signature here, obviously.)
Preview can store multiple signatures, so if you need to both sign and initial documents, you're able to do so easily using Preview's annotation functions. It's a great feature, and one that ensures my printer will be gathering even more dust than it already has.
[Just to be clear, this process only applies a graphical representation of your signature; it does not cryptographically 'sign' the PDF document to ensure that it has remained unmodified. Adobe's Acrobat application can sign PDFs with both a graphic and a digital signature; NitroPDF also has this feature, as does the DocQ web service. The DocuSign web service provides 'electronic signatures,' which are not exactly the same thing either. –Ed.]
One step closer to a truly paperless office