Faxzero is a free service that is advertiser supported by putting an advertisement on a coversheet that the recipient sees. You cannot receive faxes with this service, but you can send up to two free three page faxes a day. For longer or advertisement free faxes, there is a charge of $1.99 a page via Paypal. Faxzero supports .doc or .pdf files or a text based message. You cannot receive faxes using this service.
The eFaxFree plan allows you to get a fax number (not always a local number) that sends faxes as an attachment in your email. You can receive up to 20 pages a month free with this service. You cannot send faxes for free using this service, but do have the option of paying for a monthly fax plan with many other features.
Drop.io is an online file service that serves as a virtual drop box for your files. Once in your "drop", you can send .doc and .pdf files to a fax number. You can also receive faxes as .pdf files to your drop.
These services are monthly subscription plans that allow users to send and receive faxes. Most of theses services give you a local or toll free fax number and allow you to manage faxes online without requiring a fax machine. Pricing, number of included pages, and other features vary by provider and plan. Please check out these services individually to see what best suits your needs.