Hello Fax enables you to sign a document using your mouse (or upload a saved .jpg file of your signature) and sign documents via email! I just tested it and it is easy to use and works like a charm. The email/fax arrived in seconds and the quality was great. I realize this is a nerdy post (like this one) but I can think of numerous occasions when this would have been a considerably easier way of signing something than finding a fax machine, particularly since, as a lawyer, we still have to fax signature pages regularly... and clients never seem to be in their offices or near a fax machine.
You can test the service for free and the first five faxes are also free. After those first five pages, the pricing places are available here.