Sign PDFs right in Mail

 If you are emailed a PDF to sign, you don't have toprint it, sign it, then scan it back in:  you can actually sign it right in Mail.
Drag the PDF into the email you're sending, hover over it then at the top right you'll see a little button appear; click it, and you get a range of Markup options, including one for signing documents. Best of all, you can either add your signature by holding a signed piece of paper up to the webcam on your Mac – and it does a great job of cutting it out of the background – or by drawing on your trackpad.
Got an iPad stylus? Try using that instead of your finger!