Well, technically you can't send faxes OVER Skype*. But the guys at Skax are linking their an upload-to-fax solution to existing Skype accounts. It's only $.29 a page and follows the Skype "credit" model. In this case, you buy $5 of credits, and as long as you use it once every 180 days the credit won't expire. Compare that to eFax, where you have to spend $16/month plus a $10 startup fee!
To use the service, you need to have Skype installed and logged into your account. You also have to be running a non-Vista flavor of Windows, and use Internet Explorer to access the site. (A pain, but hey, it's in beta). They'll give you $1 of credit just for showing up, so it's a great way to send a quick free fax.
* mainly because of compression... learn more here.