I've had an eFax number for quite awhile now, and jFax is basically the same company and service just a different brand, now we can add faxZero to the list of facsimile resources.
With zeroFax you can send faxes for free, but they will include an ad on the cover page. There's a limit of sending 1 document with a maximum of 3 pages. You can send up to two faxes a day for free.
If you'd like to pay for the premium option at $1.99 per fax, payable via PayPal, you get no ad on the cover page, a single fax can have up to 15 pages and you can have unlimited daily faxes. Much more features but $1.99 per fax seems a bit steep still.
This is an area where I thing a big player like Google, Microsoft or Yahoo could come in and let people add a local fax number to their e-mail accounts and include ads in the interface of their e-mail when a fax is received.
Another free fax service to check-out is called GotFreeFax.com. Free, but unlike other similar services they do not add any branding or ads to their users faxes, making it ideal for sending formal documents if needed. Users can also receive faxes for free with GotFreeFax.com.