I have an old eFax phone number from back when they were free, but they now charge to send, so I use DocHub
for outgoing faxes. The best thing about DocHub is their Google Docs integration, and that DocHub can add fillable fields to any document for you. I tend to fill out forms there, and then fax them from there if I can't email them for some reason.
EDIT: I just checked, and they don't seem to offer any free outgoing faxes any longer, or allow you to pay per fax, it's just a monthly plan of up to 35 outgoing faxes, but that's only $4.99. Still, I liked it better when they charged you $1.99 for each outgoing fax, as I usually only need that once or twice a year, if that.