1 - Venus transit observed in 2004

2 - Venus Transit 2012 at Mauna Loa Hawaii as predicted by National Solar Observatory

Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874 and 1882). 
The last one occurred in 2004 June 08 and the next transit of Venus will occur on 2012 June 06.

Transits of Venus come in pairs separated by approximately 8 years. These pairs are then separated from each other by better than 100 years. Before 2004, the last transit was in 1882. After the transit of 2012, the next won’t occur until 2117.

By June 2012, as Venus leaves the evening sky to enter the morning sky, this brightest of planets will pass right in front of the sun, to stage one of the rarest of predictable astronomical phenomena: a transit of Venus across the sun’s face. This upcoming transit of Venus will be the last one for the 21st century. It will take place across a period of nearly 7 hours on June 5-6, 2012. During the transit, Venus will appear in silhouette as a small, dark dot moving in front of the solar disk. This exceedingly rare astronomical event – a transit of Venus – won’t happen again until December 11, 2117.

The transit starts when Venus’ disk is externally tangent with the Sun (ingress, exterior). “From then, the planet may be discerened as a little black dent in the solar limb, gradually growing bigger until the entire planet is seen on the solar disk (ingress, interior). 
During the next five to six hours, the planet will traverse the Sun’s disk until the planet’s disk will touch the opposite solar limb (egress, interior). 
The transit ends when the planet’s disk is externally tangent with the sun (egress, exterior).”
you may find your local transit times here :http://transitofvenus.nl/wp/2011/07/28/find-your-local-contact-times/

This year the transit will last 6 hours 49 minutes and 40 sec

“On June 3, 1769, the planet Venus briefly passed across the face of the sun in a cosmic alignment that occurs twice per century. Anticipation of the rare celestial event sparked a worldwide competition among aspiring global superpowers, each sending their own scientific expeditions to far-flung destinations to time the planet’s trek. These pioneers used the “Venus Transit” to discover the physical dimensions of the solar system and refine the methods of discovering longitude at sea.”

“The long wait for a transit of Venus finally ended at 3:06:22.3 PM Honolulu mean time, December 8 1874, when George Tupman became the first person in 105 years to see a transit of Venus. He had two advantages that gave him a head start: the Hawaiian stations were the closest in the world to the Delislean point of earliest ingress: and he was observing with a spectroscope that allowed him to spot Venus against the sun’s inner atmosphere, the chromosphere, a full 39 seconds before it touched the visible solar limb.”

(from var. internet sources)