This one is not a spectacular but its interesting because it's rare and because of history. Specifically, in old times (pre 1900s), people used this rare event to measure distance between Earth and Sun. Infect astronomers didn't knew this distance from a long time, so they invented so-called "1 Astronomical Unit (AU)" when they want to refer to this distance. In old times, everything was measured relative to AU, for example Venus is 0.7AU away from Sun. Venus Transit helped to resole this. Plus it also helped to figure out Sun's diameter. Now just out of curiosity, I thought, how would Venus transit could help in determining Sun-Earth distance and Sun's diameter? Answer is simple enough: Have two people watched it from different latitudes on Earth, make a triangle, use simple trigonometry and you get the distance! Nasa has put up a detailed explanation of little math in involved in this clever technique. Then This is, of course, still less accurate then modern ways of radars and satellites. Venus orbits around Sun in just 0.6 years. Then one would think why transits are so rare? Well, answer is, Venus's orbit isn't in the same plane as Earth, so it's pretty rare for Sun-Venus-Earth to get in one straight line, usually every little over hundres years or so. Apart from that, this transit won't visually appeal to most but if you had those special safe glasses to view sun eclipse, you might have a look between 11:05AM to 11:25AM on US East Coast on June 8, 2004. More info at Exploratorium and astronomy.no.