Here are some examples:

* Bacteria: These are prokaryotic cells, meaning they lack a nucleus and other membrane-bound organelles.

* Archaea: These are also prokaryotic cells, but they are distinct from bacteria in their biochemistry and genetics.

* Protists: These are a diverse group of eukaryotic cells (meaning they have a nucleus and other organelles) that includes amoebas, paramecia, and algae.

* Yeast: These are single-celled fungi.

Unicellular organisms are incredibly diverse and can be found in almost every environment on Earth. They play vital roles in ecosystems, from decomposing organic matter to producing oxygen.