There are also email-to-fax services, if you don't want to put another app on your phone. eFax happens to do it that way too. I haven't found any free unlimited ways to send a fax from cellphone, but there are ways to send and.or receive a couple a day free.
If you want a reliable outbound faxing service on your phone for occasional use, you might want to try ScanWritr. You simply load it with fax tokens and you're ready for the time you might need it.