Ground-state calculations based on current spin density-functional theory for circular parabolic quantum dots are presented which reveal interesting spin effects in dependence on an external magnetic field: Hund's first rule (well-known from atomic physics for zero magnetic field) is related to the level occupancy near the Fermi energy and determines the spin configuration of the ground state in finite magnetic field. Consequently, the addition spectrum of the quantum dot exhibits a specific non-monotonous dependence on magnetic field, which is visible in recent quantum transport experiments.