Galileo is best known for its reconnaissance of Jupiter. As it transited the asteroid belt en route to Jupiter, though, it made the first close flybys of asteroids, passing within a few hundred miles of Gaspra and Ida. It discovered that Ida has a tiny moon, which later was named Dactyl. In 1994, Galileo provided the only direct look at the impacts of Comet Shoemaker-Levy 9 on Jupiter.
The most extensive study of any minor body to date was provided by NEAR-Shoemaker, which entered orbit around the asteroid Eros on February 14, 2000. It transmitted thousands of images of the asteroid plus extensive measurements of its shape, composition, and gravity. Although it was not designed as a lander, as its fuel neared an end, flight controllers maneuvered it to a gentle touchdown on the asteroid's surface on February 12, 2001 -- the first landing on any asteroid or comet. The craft's instruments transmitted data from the surface for several days.
In 2004 and 2005, three spacecraft upped the ante in the exploration of the solar system's minor bodies. Stardust flew through the coma of Comet Wild 2, gathering grains of comet dust that it returned to Earth in early 2006. Deep Impact fired a probe at Comet Tempel 1, gouging a fresh crater and giving astronomers a look at material beneath the surface, which probably had remained unchanged since the comet's birth. And Japan's Hayabusa orbited asteroid Itokawa, landed on its surface, and gathered a tiny sample of material for return to Earth in 2010.