I've been using pamfax
http://www.pamfax.biz/en/ which works out to 8.4p per fax to most numbers in the western world.
It basically works just like a printer once installed. No hassle. No advertising on the recieved fax, either.
The big plus of the system is that it's totally internet based - no phone line or modem is involved at all.
If you have a skype account, you can use your skype credit for the faxes. In that case, there's no need to buy an initial £10 of credit.
For me, I only need to send faxes from home very occasionally and never need to receive them - so this works out fine for my needs.
I think you get a free fax when you first try it out too, which might be useful if you're in a bind and just need to send one fax for some reason.
Cheers,
Ty