I know this is an old thread, but just in case any Mustachian searches for an internet fax service, I wanted so share my discovery.
I have used faxaway.com for at least five years.
It costs me $1/month for my own fax number, plus $0.11 minute to send or receive faxes. The $1/month "maintenance charge" is actually relatively new (about two years ago they started it, before that, it was free for any month I didn't send or receive any faxes).
So, a typical month for me now costs $1. A month where I fax about 20 pages is about $2.
Before I posted this, I read at the bottom of "MMM recommends" that MMM recomends Maxemail, at a cost of $24/year or more, so I thought this faxaway option was worth mentioning, since it could represent a savings of more than 50% for most light fax users like me.