OS X: Need to sign a PDF but don't have a pen? If you've got signature, you can use your finger and multitouch trackpad to sign it digitally.
Signing digital documents has gotten a bit easier over the last few years, especially on the Mac, with great apps like Autograph and the built-in signing option in Preview, but both have their pros and cons. Autograph, while quite good, can't store signatures and has been a little unstable in recent releases. Preview, on the other hand, requires a webcam and an image of your signature.
The quality of the final product doesn't look so great, either, but you can store signatures so you don't have to scan them in all the time. Signature, however, combines the best of both: you can sign with your finger on the trackpad and store signatures for later use. Of course, this presents a mild security risk so you can choose to delete them after use. If you lock your computer, however, there isn't much to worry about.
Signature hides up in your menubar and doesn't bother you unless you activate it by clicking or with a keyboard shortcut. From there you just sign on your trackpad and copy the signature to the clipboard so you can paste it into a PDF. Alternatively, you can just sign and save that signature for later use.
Signature ($2.99) [Mac App Store]