While we're talking about free fax services...
Others have already mentioned the outbound free fax service provided by tpc.
It is also possible to get free inbound fax from either
I forget which is which, but one has a free service (works via a national rate number, so some people/companies may not want to fax to it). Also I think the level of spam into my wife's account increased when she subscribed.
There are doubtless other companies doing this, but this is the only free one i've found.