The architectures of extrasolar planetary systems provide key properties that trace the processes of planet formation and evolution. Until recently, Jovian-size planets provided nearly all of the observational constraints. I will describe two new probes of the properties of small (sub-Jovian) planets. First, using Doppler measurements from NASA-UC Eta-Earth Survey at Keck, our group measured the mass function of planets from Jupiter-mass down through super-Earths. This mass function rises steeply toward lower mass and, extrapolating down to one Earth mass, we predict that one in four Sun-like stars have a close-in planet of approximately Earth mass. Second, the distribution of close-in transiting planets detected by the Kepler space telescope paints a similar picture of rising numbers of small planets. The Kepler discoveries also demonstrate that small planets are more common around low-mass stars and at greater orbital distances. My talk will describe how these patterns of planet occurrence, including the notable absence of the predicted desert of close-in low-mass planets, offer key constraints on theories of planet formation and migration.