Here are some examples:

* Bacteria: These are tiny single-celled organisms that are found everywhere on Earth.

* Archaea: These are similar to bacteria, but they have different chemical makeup and are often found in extreme environments like hot springs or salty lakes.

* Protozoa: These are single-celled organisms that are usually larger and more complex than bacteria. Examples include amoebas and paramecia.

* Algae: Some algae are unicellular, while others are multicellular. These organisms can produce their own food through photosynthesis.

Unicellular organisms are incredibly diverse and play important roles in ecosystems. They are responsible for everything from decomposing organic matter to producing oxygen.