I have been happy with Faxaway. I think my $10 payments each last me almost a year, given how much I use the service ($1/month plus $0.11 per minute sending in U.S., receive faxes for free). The interface is email: put fax phone number + '@fax2.faxaway.com' as recipient, 'FAXDOC' as subject, and attach pdf of document you're faxing. Works great.