Rendering of a cryovolcano.
Evidence for cryovolcanic activity has been seen in many of the coldest moons in our solar system, including Triton, Ganymede, and Titan. Generally, gravitational tidal forces create enough heat in the core of these moons through friction to melt volatiles. These volatiles, usually liquids, like water, ammonia and methane are forced to the surface by the planets internal pressures. Collectively, these volatiles are called "cryomagma". This cryomagma usually freezes when it leaves the surface and is exposed to the extremely cold temperatures of the planet's surface. Tidal forces aren't the only thing that can melt these volatiles however, radioactive decay could theoretically create enough heat for cryovolcanism. Hypothetically, Quaoar in the Kuiper belt has shown cryovolcanism in the past which could be explained by radioactive decay.
Another rendering of a cryovolcano
The first conclusive evidence of cryovolcanism is Sotra Facula on Titan. Saturn's largest moon, Titan, has a heavy methane atmosphere. The thing is, solar winds are constantly blowing this atmosphere away. It is theorized that cryovolcanoes might replenish Titan's atmosphere by bringing methane from the moons interior. We now have pictures of a particularly prominent feature of Titan, Sotra Facula, which appears to be an eruption crater. It is a cryovolcano, the first one that we actually have a picture of.
Sotra Facula. The green areas are thought to be volcanic, blue areas are sand dunes.