Ask HN: Faxing without a fax machine?

I've been freelancing for about 10 months, and occasionally the need arises to sign or fill out various documents emailed to me. I don't have a land line since I only use my cell phone. So having a fax machine isn't viable. Driving to someplace like Kinko's is a pain and expensive. Paying for a fax service doesn't really make sense since I don't fax that often.

Does anyone know of any good solutions (or apps) that can help with this?