Streptavidin is a tetrameric protein purified from the bacterium Streptomyces avidinii. It has an extraordinarily high affinity for a molecule called biotin (also known as vitamin B7);
Streptavidin is used extensively in molecular biology and bionanotechnology due to the streptavidin-biotin complex's resistance to organic solvents, denaturants (e.g. guanidinium chloride), detergents (e.g. SDS, Triton), proteolytic enzymes, and extremes of temperature and pH. Essentially you can use it to attach different molecules to each other in a specific way like attaching pieces of velcro to things.
Traptavidin (Shown above) is a human engineered mutant of Streptavidin with an even higher binding affinity (10x better) showing that we're now able to improve and modify natures machines for our purposes.