[Fixup]

box.net

You can upload a document or picture, edit it or not, and fax it. Free. works in opera mini. if you need to edit it, then use Opera.
of course you can do it on PC as well.

fax1.com

You can print any document to a virtual printer and it'll fax over internet. You can also send an email with or without attachments and fax1.com will fax it for you. Not free though but a $5 deposit lasts forever if you don't fax too much.

Then buy a ordinary fax machine for you parents.