The new Apple OS upgrade (named: Lion), which will be released in July, has an interesting addition for people who've wanted to use digital signatures to sign electronic documents, but couldn't figure out how to scan in their signature. From Apple's info page, comes this tidbit:
Add your signature
Preview makes it easy to add your signature to PDF documents. Hold your signature up to an iSight or FaceTime camera, and Preview uses sophisticated image processing to turn it into a pen-perfect signature annotation. With a click, the signature is added to your document, which can be saved or sent via email. It’s ideal for signing PDF forms and documents.
P.S. If you want a better practice, check out this Ultimate Guide.