The easy/obvious answer is all of them. SO TRUE!!! But for online faxing, you really can't beat
GreenFax or
eFax, two excellent choices that combine value, flexibility and feature rich platforms.
eFax is particularily compelling if you don't plan to fax more than a couple pages a month because they offer their services gratis (free) for less than 20 pages. It's worth noting that in all my dealings with these two companies, neither has ever charged me erroneously, nor commited acts which I considered to be financial fraud. Unfortunately,
docuharbor has, and I consider them to be an unethical, sketchy online fax company. But that's just my opinion and I only used them for 56 days....while paying for 90. eFax is worth monitoring though, because their TOS could have them end up playing the same "gotcha" game that
Docuharbor.com played with me
In all honesty, your best option is almost always setting up your own fax machine/line with your local telco. Their rates are getting more and more competitive and they don't have the same kind of ability (or interest) to assault your good senses and abuse your trust.
A great resource for online faxing reviews and your online faxing needs is this link:
Fax Roundup. Except that that guy has referral links, as well as sponsored links, so he probably is completely biased and bought like a monkey in a suit.
Two even better resources are the tech bargain sites,
Techdeals.net and
Techbargains.com, wherein you can find listings for deals on hardware, and, occasionally, fax machines. Very pro-consumer and I've honestly saved thousands of dollars over the last 4 years that I have used them. Just remember to buy what you need -- you aren't saving money if you are getting a great deal on something you will never use.