I never did any sending of faxes online and only received them.
Callwave service supplied me with a free fax number and a small tray app that notified me of incoming faxes (and voicemail) and allowed me to download them as pdf files.
They have since
partnered with
efax and I don't think they have the tray app that alerts and keeps a record of incoming faxes. (I could be wrong, though) It's not a free service any more and it's also 2-way now, with both sending & receiving faxes by email.
efax has also partnered with OpenOffice and you can now get a plugin to send faxes from within OpenOffice by just clicking a toolbar button. (again, not free)
They also have an additional application that allows you to create, print, and send faxes with text notes, highlights and a signature stamp.
http://www.efax.com/products/internet-fax