Here are some examples:

* Bacteria: Many types of bacteria, like Clostridium botulinum (which causes botulism) and Bacteroides fragilis (found in the human gut), are obligate anaerobes.

* Archaea: These single-celled organisms are often found in extreme environments like hot springs and deep-sea vents, where oxygen is scarce.

* Protozoa: Some protozoa, like Giardia lamblia (which causes giardiasis), can survive without oxygen.

It's important to note that some organisms can tolerate low oxygen levels but prefer environments with oxygen. These are called facultative anaerobes.