The next shadow to cross the Sun will be that of the planet Venus. This event is similar to a solar eclipse however the shadow is a mere dot. At closest approach Venus is a mere 1/32 the size of the Sun when viewed from the surface of Earth.
The image below shows Venus crossing the Sun in 2004. A nice round dot against the solar surface. A Coronado hydrogen alpha telescope (40mm) was used with a digital camera (Canon) and Barlow lens to capture the image.
The last transit of Venus visible from Earth was in 2004. Prior to that, the last transit was in 1883. There were NO transits of Venus during the 20th century. That means no astronomer alive today had seen one prior to 2004.
Here we are, eight years later and another one is about to occur. That doesn't make them seem rare until you think about the fact that the next one will not be visible until the year 2117. That is over 100 years from now!
So if you are able to see it, do so. It is a once in a lifetime event (twice if you saw the 2004 transit). To find out what time it will be visible, visit the following web calculator site for the When given a location.
More information about the Transit of Venus can be found at: http://transitofvenus.nl/wp/ and http://transitofvenus.org/