There’s plenty of software programs on the Mac App Store that will allow you to add a signature to a PDF, but come Lion, you won’t need any of them: Preview.app will do it by creating a perfect digital pen signature via FaceTime.
Here’s how to sign a PDF in Preview under OS X Lion.
What I do now is scan a signature. Open up the file in Photoshop along with the signature page of the PDF. Copy the signature over to the signature page and format. Save the page as a PDF. Open Preview with the PDF containing the unsigned signature page along with the new PDF of the signed page. Move the signed page over to the other document and replace the unsigned page by deleting it. Now I can save the document that has the signature.
With Lion, I can do it all in one app. Sweet.