Be careful with the idea of "HIPAA Compliant Fax". The Department of Health and Human Services (HHS, the organization behind HIPAA) does not certify any technology as being "HIPAA compliant". It's up to the organization that is affected by HIPAA to be "compliant" by choosing technologies and operational practices that all it to do so.
That said, when it comes to any Internet fax service, there is a lot of misinformation out there about HIPAA compliance:
1) Any service that uses email to send or deliver faxes is most likely not HIPAA compliant. That's because unencrypted email is not HIPAA compliant. There are several ways to solve this, but few providers offer any of these solutions "out of the box". One simple solution is to only use a web portal or client software over an encrypted connection (of sufficient strength) to send or receive faxes. Some providers offer this, but you must make use of the feature (i.e. rather than using "clear text" email).
2) Further on the point of email, if faxes containing HIPAA-governed content gets stored in your email, then your email system (or provider) must maintain proper security as well.
3) Another key point is how faxes are stored on the provider's servers, whether they are only stored temporarily or stored for long-term archival. Are these files encrypted, and how are they encrypted? Who has access to these files? What security and business process policies are in place? This is much harder to determine, as it's not typically something the provider will include in enough detail on their website. You need to ask.
4) Is your provider willing to sign an HHS Business Associate Contract? If not, you can't really be sure they they will maintain the necessary security and business practices necessary for you to main HIPAA compliance.
You can learn more about HIPAA here: http://www.hhs.gov/ocr/privacy/hipaa/understanding/index.html
FaxLogic is one provider that meets the requirements necessary for you to be HIPAA compliant, and will enter into a Business Associate Contract with you. Learn more about FaxLogic here: http://www.faxlogic.com