While this is slightly debatable due to the Idea that the Universe is Infinite it is rather unlikely to find a Moon as large as ours around a planet similar to Earth. From our Understanding of how Planets form the Terrestrial planets (rocky planets like Earth) do not form with Moons like the Jovian planets (large gas planets). This is due to the fact that the Terrestrial planets are far less massive than the Jovian’s and also due to the small percentage of solid material out of which to form the Moons. However the biggest reason why it is Unlikely to find a Earth-like moon around a Terrestrial planet is due to the Unique origins of our Moon. As you hopefully read in my little narrative (or can easily read about elsewhere on the web) the Earth’s Moon was formed by a collision between our early Planet and a Mars like planet. This unusual Origin is why it is unlikely to find a similarly Large Moon around a planet similar to Earth.
As you more than likely know the Moon’s gravity pulls the Waters of the Earth toward it causing Tides. Of course the Earths gravity also Pulls on the Moon only since the Earth has more gravity it actually causes the rock in the Moon to shift and bulge toward the Earth. Over millions of years this pull on the Moon has caused a sort of friction which caused the Moon’s rotation to slow such that only one Face points to the Earth at any given time.
Ok, I admit this Question is a little ambiguous, but Rings works better than most answers. You see Planetary Rings are caused by Moon’s being torn apart by planetary gravity. The remains of the Moon are the debris we see as Rings.
One of Jupiter’s Moons, Io was expected to be a rather dead and uninteresting moon, however pictures from the Cassini mission provided pictures which show many active Volcanoes on Io’s surface. It is believed that the planet is heated through Orbital friction in a process known as Orbital resonance. Io’s orbit aligns with some of the Outer moons of Jupiter periodically and the additional gravity causes frictional heating of Io’s interior.
One of Saturn’s moons, Titan has approximately 1.5 times the atmosphere of the Earth and is larger than the Planet Mercury.
Another of Jupiter’s moons, Europa has liquid Water under its icy surface and this water makes it the best place to look for Life in our solar system.
Due to its Angle with the Sun when the Moon rises at Noon and sets at midnight it is in its first Quarter phase.
Similar to why the Sky is blue, the Earth’s atmosphere scatters blue light but lets red light pass through easily. This is the same reason why Sunsets and Sunrises appear to have a reddish color.
The current count is over 100, though many of the moons are rather small.
Also known as the Galilean Moons, these were the first moons discovered beyond the Earth’s Moon.