An Australian company called Solar Sailor is designing free standing "sails" (they really look like airplane wings) that can be folded down when not in use. The sails are covered in solar PV so they generate electricity for the hybrid diesel-electric drive as well as providing lift when the wind conditions are right. One of their first vessels is a passenger ferry in Hong Kong.
As a disclaimer: This technology is not very widespread or economically viable (yet).
The company hopes to expand their business in China to bulk carriers transporting iron and other metals, but they have not yet built a prototype at that scale. Their viability may increase soon as shipping companies need to meet new energy efficiency and sulfur dioxide emissions regulations set by the UN International Maritime Organization.
Hopefully this technology will be economically viable soon, but until then, it's pretty darn cool.