Only exception was Saturn's moon Titan, which in 1944 was discovered to have an atmosphere. Then in 1979, the Voyagers flew past Jupiter, and photographs of the moon Io showed not only a very young surface, and many volcanoes, but even actual plumes from volcanic eruptions!
Since then, slowly but steadily, many other objects turned out to be more interesting than ever imagined, with ongoing geologic processes, or some form of meteorology at least. Let's list them all!
Io also has a varying atmosphere, made up mostly of sulfur dioxide. It's very much dependant on the volcanoes, that supply the otherwise slowly escaping gases. Jupiter's magnetosphere sweeps up gas and dust from Io's atmosphere and they up in the radiation belts around the planet. See an article about Io's atmosphere (the "Ioan" atmosphere?).
Europa is thought to have a 10-30 km thick crust of water ice, which hides a (probably) 100 km deep ocean. Tidal heating is keeping it from freezing, and drives the geological processes. Crater counts lead to an estimate that the surface is less than 30 million years old. Some think that its long ridges are the ice based analogues of Earth's oceanic ridges, ice/water volcanoes spreading the crust.
Ganymede, the largest satellite in the solar system, which is actually larger than Mercury or Pluto, shows extensive signs of tectonic processes. Its surface is a mix of two types of terrain: very old, highly cratered dark regions and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges. Ganymede is not thought to be active any more. However, it is the only moon that has magnetic field of its own. It is not fully understood yet, but it may be generated in a similar fashion to the Earth's, by movements in a molten metallic core.
Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years. Callisto experiences the least tidal heating from Jupiter. However, high resolution images from Galileo show deformed craters, and a lack of small craters. Examination of the surface shows that there has been gradual slumping, or "relaxation" of the craters, and what is termed "sublimation-erosion" of the surface.
See JPL news article.
Titan is the only moon with a real dense atmosphere. It was long theorized that methane and ethane may form clouds, fall to the surface as rain, and create rivers, lakes or oceans. The Cassini spacecraft did indeed discover what appear to be lakes, near the southern and northern polar regions. The moons surface is formed by the atmospheric processes, probably wind, rain, and maybe streams. Ice-volcanic activity is also suspected, but hasn't been confirmed yet. It looks like the basic processes are analogous to Earth's, with hydrocarbons taking the role of water, and ice/water taking the role of rock.
In 2005, the Cassini probe observed a localized atmosphere at the south pole of Enceladus, through a stellar occultation, and also from particle measurements. It also imaged a hot area, in that region, with its infrared spectrometer. Later it was able to directly observe the water vapour plumes emanating from the southern parts. (see here)
Geysers that spew ice/water would mean that there's a source of liquid water. Is that possible at -200 °C that is the observed temperature of the surface? Well, the depths of Enceladus may be heated by remnant radioactivity or tidal heating.
However, another hypothesis was also put forward, which doesn't need a liquid layer, rather, simply the (violent) decomposition of clathrates (see article, and the original in Science).
The plumes of Enceladus are also replenishing Saturn's E Ring with tiny particles.
Researchers are now looking for possible geysers on Saturn's smooth icy moon Dione - which appears to be a less active version of Enceladus (see article and
Iapetus one of the weirder bodies in the Solar System. It's not just its weird equatorial ridge. It's not just that it's two-faced, with its leading hemisphere as dark as soot, and its trailing hemisphere among the brightest. It's even more intriguing that there's no grey part at all - no transition between the two sort of covering, as far as the highest resolution images show.
NASA scientists think this is a result of thermal segregation - a process in which the slightly darker ice absorbs more sunlight, thus it sublimates, only to become trapped on the colder surface of slightly brighter ice.
Researchers are now looking for possible geysers on Saturn's smooth icy moon Dione too - which appears to be a less active version of Enceladus (see article and
Ariel, a satellite of Uranus, shows a young surface, with few ancient craters, and the existing ones distorted. It also has large faults, canyons, with smooth floors. It certainly underwent considerable activity in the past. A theory is that the driving force behind this was tidal heating, due to a different, more eccentric orbit in the past. Ariel has high water content, and the highest reflectivity of the Uranian moons. Can it be a result of fresh ice crystals on the surface, from cryovolcanism still going on? We don't know yet.
Other Uranian moons, Miranda and Titania also show a relatively young surface, compared with the other large moons in their neighborhood.
Triton is Neptune's largest moon. It's the 7th largest satellite in the Solar System, with a diameter only 20% smaller than our Moon's. But it is much more alive than the Moon! It has a very young surface, it has polar caps, it has a very tenous atmosphere (estimated to have surface pressure of 0.01 mbar). A tenous atmosphere which nevertheless is enough to hold an observable cloud - which is created by cryovolcanism, as directly observed by Voyager-2!
Movie showing geysers on Triton
Pluto's atmosphere was detected through stellar occultations. It varies considerably with time, as most of this atmosphere is thought to freeze out onto the surface as Pluto reaches its farther parts of its orbit around the Sun. However, recently, some spikes were observed in the density of the atmosphere, even though Pluto is right now receding (see article).
Unlike Pluto, its moon Charon has no atmosphere. However, it appears to be host to cryovolcanism, or ice geysers (see New Scientist's article, and original article). The source is probably a liquid water-ammonia layer.
The presence fresh water ice crystals on the surface of Charon are inferred from its spectra. However, some of the Kuiper Belt Objects, recently discovered, appear to share this spectral feature with Charon. Its not impossible, that some of these objects also have cryovolcanic activity.