Ranunculaceae: Buttercup Family
This family has about 2,000 species in 50 genera. They are often called the herbaceous counterpart to the Magnoliaceae because of the relatively simple structure of their flowers.
Like the magnolias, the buttercups represent a family of plants that have relatively unspecialized floral parts. Some taxonomists refer to families like this as primitive. However, it is important to remember this is only because they have retained their ancestral characteristics, not because evolution passed them up. In fact, many of these species appear to have coevolved greatly with pollinators (such as the Columbines). But, because all floral parts attach independently these plants are considered simple.
CAREFUL: Many of the species in this family are considered extremely toxic. The most poisonous plant in the world is a type of monks hood (Aconitum) from Nepal. Handling or smelling this plant can cause unwelcome symptoms. The North American (Aconitum columbianum) is also very dangerous. Because of the toxic alkaloids found in many of the plants in this family, it is best to avoid eating them unless you're with an expert.
Some of the most showy plants in this family are the Larkspurs (Delphinium sp) and Columbines (Aquilegia). They are usually found in the temperate regions of the northern hemisphere.
Systematics: As T.J. Elpel once wrote, "the most common pattern of the family is the apparent lack of pattern" (Elpel 2004). Its true. Sometimes the flowers are regular, sometimes irregular. Often they are bisexual. Probably the most reliable identifier is to look for multiple simple pistils. Only a few of the more "advanced" species have pistils that are fused into one. Also, look closely and you might see that some of the pistils have hooked tips.
Additionally, one might say they are arranged in 2, 5-merous cycles and the petals are nectariferous near the base.
Links to more information
Current Recognized Genera (compiled via Wikipedia & Elpel T.J.)
Aconitum - Monk's Hood, Wolfsbane