You can make signatures in Preview that are pen and ink style drawings.

I don’t know if this has been posted yet, or if it is generally known, but I stumbled upon a neat feature of signatures in Preview.app. When you create a signature, if you you don’t hold up a piece of paper with a signature on it, and simply smile for a mug shot, Preview will create a signature that is a neat pen and ink style image of yourself, or whatever is in front of the camera. I’ve created several, that for the right client, can be used as a humorous alternative to an actual signature. I also used a screen capture of the signature on a document and made the image into a Facebook Profile picture.

[crarko adds: Well I didn’t know about it. Most folks use Photo Booth to do things like this, I’d guess, but I always appreciate a creative use of a program.]

Source: Mac OSX Hints