Living things that have only one cell are called unicellular organisms. Some common examples include:

* Bacteria: These are the most common type of unicellular organism. They can be found in almost every environment on Earth.

* Archaea: These are also single-celled organisms, but they are distinct from bacteria in their biochemistry and genetics. They often live in extreme environments, such as hot springs or salty lakes.

* Protists: This is a very diverse group of single-celled organisms, including algae, amoebas, and paramecia.

It's important to note that there are many different types of unicellular organisms, and they can be quite complex despite their small size!