Bacteria:
* E. coli: A common bacterium found in the gut, some strains can cause illness.
* Staphylococcus aureus: A common bacterium on skin, can cause infections.
* Streptococcus pneumoniae: Can cause pneumonia and other respiratory infections.
* Cyanobacteria: Photosynthetic bacteria that were vital in oxygenating the early Earth.
Archaea:
* Methanogens: Produce methane as a byproduct of their metabolism, found in swamps and the gut of animals.
* Halophiles: Live in extremely salty environments, like salt lakes.
* Thermophiles: Thrive in hot environments like hot springs and hydrothermal vents.
Protists:
* Amoeba: Move using pseudopodia (temporary extensions of cytoplasm), some are parasitic.
* Paramecium: Have cilia for movement, some are predatory.
* Euglena: Photosynthetic protists with flagella, can switch between heterotrophic and autotrophic lifestyles.
* Diatoms: Have intricate glass shells, form a large part of phytoplankton and are important for ocean ecosystems.
* Plasmodium: Parasitic protist that causes malaria.
Fungi:
* Yeast: Single-celled fungi used for brewing, baking, and other industrial processes.
Note: There are many other single-celled organisms out there, and the boundaries between different types are sometimes blurred. This is just a small selection to give you an idea of the diversity.
