RingCentral Fax. This service is one of the leading ones in the field of online faxing and is 100% compatible with Google products (inlcuding their own Google Docs add-on).
eFax. With this service you can be faxing within minutes from anywhere. With presence in dozens of countries around the World, you could say that eFax is the biggest online fax service in the market, and it offers the best mobile fax app out there.
MyFax. This company offers basic faxing capabilities at a great price. If you prefer a more affordable solution, MyFax is what you are looking for. Besides that, they offer an impressive list of compatible file formats that you can fax online!