Yeasts are a kind of single-celled fungi that ferment sugar. They do not correspond to a single taxonomic group, but the subphylum Saccharomycotina, sometimes called the "true yeasts", is probably the most interesting. Saccharomycotina includes baker's yeast and Candida.
Yeast gets my vote for number one microbe of all time, being critical to many fermentation processes, including the making of bread, beer, and wine.