The built-in Mac OS X Preview app is designed to handle a lot of different kinds of files ranging from images to PDFs. If you happen to be using it with a PDF file it has a lot of nice features for marking up the document, but one feature that tends to get overlooked the most is its ability to capture/digitize your signature using the iSight camera included on your Mac.
If you want to have the Preview app store your signature just open Preview up, navigate to the Preferences, and go to the Signatures tab. Then click the plus sign to add a signature, and you should be presented with a screen similar to what is pictured above. Now all you have to do is sign your name on a white piece of paper and hold it up to the iSight camera. Once you have it lined up along the blue line you should see the signature capture preview update in real-time. If you quickly pull the paper away it will leave the last successful preview on the screen, which means you don’t need to try and click Accept while holding the paper at the same time.
One tip I have when doing this is to write your signature as big as possible, and if you use a pen or marker that makes the lines more prominent your digitized signature will turn out to be even better.
Once you’re ready to use the signature you’ll need to go to the View menu and select Show Annotations Toolbar if it’s not already visible. You should then see an icon on the toolbar for the signatures, and if you click it you’ll see a list of signatures that are available. Select the one you want to use, and then click anywhere in the document to have it inserted. You can then resize it as needed. You’ve now signed your PDF for free without having to print anything out!