As of 2005, the smallest known cell to man is the SAR11 micro-organism, according to Red Orbit. The SAR11 cell measures a mere 1 micron in length.
The SAR11 micro-organism is a form of bacteria, and it is the most common organism found in seawater. These bacteria have been on Earth for a billion years, and they survive in extremely harsh conditions that kill most other life forms. SAR11 was first discovered by OSU scientists in 1990. SAR11 is an important cog in the carbon cycle. It consumes organic carbon and provides nutrients to algae, which in turn produce half of Earth's oxygen.