Kevin Savetz has compiled and maintains a list of commercial and free faxing services that that you can use via email or the web (yes, eFax is one of them). His list includes international sites as well as U.S. domestic.
A couple of note here:
eFax (free and commercial)
Fax1 ($1 credit when you sign up, no monthly fee, $0.12/page to the U.S.)
MaxEmail (commercial but it only costs $14.85/year for the basic service)