Electronically sign PDF?

I'm running a home office and looking to keep it office paperless. Printing and scanning PDF agreements and forms is tedious - any ideas what I can use on my Mac?

UPDATE:

I googled around and could only find PC software until I searched on the appstore and found PDF Signer. I tried the demo and it's just what I was looking for. It even has the option to add text fields to complete any empty spots on an agreement, and it allows me to save the new information back to the PDF.

Let me know if you've heard of something similar for free (although $10 seems very reasonable...)