I have used Fax Zero for the very few times I have to send a fax (usually, some sort of document copy to a banking institution). I have used both the free and the paid-for service, and was very satisfied with both.

First of all, the free fax they offer is 1)truly free 2)works well and fast. There's a 3-page limit per fax/2 faxes a day bound (as of 2011), but that's usually more than enough. Most pleasantly, there is no sign-up, no clikcking, no other conditions: you just select a document to send, type in name/number/etc., click, and you are done (well, you have to click once more in the confirmation email). All other services I know of require an account, but this does not.

The free fax does include a Free Fax logo on the cover page ("advertisement") to let people know  that their service was used. I think it's no worse than a "sent from my iPhone" in the bottom of an email, which people put voluntary.

The non-free fax is very cheap,  at $2 IIRC, allows you to remove/customize the cover page, and has a 15 page limit. It is faster than the free fax (I don't remember how fast, but it was sooner than 5 minutes to receive a confirmation). The free fax is within 30 minutes, but, in practice, it takes much less time. In particular, a fax I sent before typing this review (which I was writing as I was waiting for a fax to be sent) has been delivered by the time I started writing the previous sentence.

Overall, I am very happy that I don't have to utilize the technology of the previous century anymore. That, and PDFescape (website which allows you to fill/sign PDF forms online)  allow one to forget that faxes exist.