I use FaxSTF on a mac - I have a mac server running in my office.
However, you don't need to even use any special software on a mac. The system software has all the tools you need to get faxes and send them automatically as PDFs to any email you select.
For the windows side, there are actually a variety of choices. The same company that makes faxstf for the mac also makes windows software. Find their stuff at:
http://www.smithmicro.com/default.tpl Once I open my office, I plan on running a lot of different services off of mac mini computers. They are relatively cheap ($600) and I can have them be web based workstations, control in-office and window advertising and be Fax and file servers.
The way I have usually had it setup is that I have a mac server that is always on working as my fax machine. When a fax is received, it forwards a PDF of it to my email address, it also can use caller ID and/or character recognition to route the fax other emails or to be backed up in a specific place. This lets me keep a great paper trail of everything that gets faxed to me or from me. Since this server is on the net, I can send a fax from that machine from anywhere on the internet.
Here is a link to the Mac Mini so you can see how small it is.
http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/wa/RSLID?mco=B3C47B0&nclm=Macmini If you were to buy a good Fax machine, with all the bells and whistles, you would pay about the same for it as you would a mac mini with the ability to have more options. Plus, you never have to pay some monthly efax fee.
R