For centuries, our knowledge of planetary systems and ideas about planet formation were based on a single example, our solar system. During the last thirteen years, the discovery of nearly 150 planetary systems has ushered in a new era for astronomy. I will review the surprising properties of extrasolar planetary systems and discuss how the wide variety of their host stars and orbital properties is reshaping theories of planet formation. For example, several mechanisms have been proposed to explain the large eccentricities typical of extrasolar planets. I will focus on how multiple planet systems can constrain these mechanisms and provide insights into broader questions about planet formation.