torque-free precession
Torque-free precession is the slow, steady wandering of a spinning body's axis even when nothing is pushing on it. Throw a spinning frisbee or a wrench in zero gravity and, if it was not set spinning exactly about a principal axis, its symmetry axis traces out a cone all on its own. There is no torque -- the precession is driven entirely by the body's own lopsided inertia.
The reason is that with no torque, angular momentum L is exactly conserved and fixed in space, but the angular velocity omega need not be parallel to L (recall L = I omega with a tensor I). For a symmetric top the figure axis, omega, and L all stay in one plane, and that plane rotates rigidly about the fixed L. Euler's equations give the body-frame precession rate Omega_body = (I_3 - I_1) omega_3 / I_1, while as seen from space the figure axis sweeps around L at rate phi_dot = L / I_1. The whole motion is two steady cones -- omega tracing the 'body cone' rolling on the space-fixed 'space cone'.
This is Eulerian or free precession, and it is genuinely torque-free, unlike the gravity-driven precession of a leaning top. The Earth does it: its rotation axis is slightly off its figure axis, producing the Chandler wobble, a free precession with a period of about 433 days and an amplitude of only a few meters at the pole. Damping in a real body slowly drives omega toward the axis of largest inertia (minimum energy at fixed L), which is why tossed objects eventually settle into a flat spin.
In the Apollo 13 films you can see slow tumbling of stranded hardware in space; a wrench floating free spins about a fixed L while its long axis nods around in a cone. With no torque to change L, the wobble simply continues, its rate set by the ratio of the tool's principal moments.
With L conserved and omega not parallel to it, the axis precesses with no external torque.
Torque-free precession is often confused with the precession of a heavy top, which is torque-driven by gravity. Here there is no torque at all; the motion exists purely because the inertia tensor makes omega and L point in different directions.