Efax has a free service, but limits you to 20 pages a month. I have used it for 5 years and had to switch email accounts only once because I hit the limit. They use a format you can't view on your BB, but you could see from the Web Client with the efax software installed on a PC.
Now with my BB email addy, I have another email addy to use if I hit the 20 page limit again