I think the most useful invention in the world so far is internet.We can search for information we need in the internet,knowing more about the world.It makes life more convenient.We can do merely every thing through internet,including ordering foods,buying books,reading,studying,talking with friends and making new freinds.Internet also enables us to express ourselves freely because the life in internet is virtual,nobody knows who you really are.
In a word,internet is the most useful invention in the world without which human can not live a better life.