I used to get the free efax deal, which was good for something like 5 faxes a month, as a trial. Then, I subscribed for maybe $10/mo. It worked well. I no longer use efax. Now, having BlackBerry, if I did still subscribe, I would be able to immediately see when I receive a fax, so it has it upside. As Mango said, you will need a scanner to scan documents to fax out. Some other software already allows you to fax out from your computer file documents.
I like to keep a digital copy of all my faxes, incoming and outgoing, so mostly, I email out, but I find that some people don't find it easy to scan a 20 page document, one page at a time, so for them, my incoming document is frequently faxed, though they don't have a problem receiving and printing the pdf which I send. My computer allows me to sign directly on the computer screen, without the need to print, a feature I really like. As technology increases and prices decrease, I think most everyone will be using that, and faxes will fall into the same room as the Beta video tape, in the Museum of Modern History in D.C.