The first time this has been seen in 122 years. On the morning of 8th June 2004 Venus underwent a transit across the face of the Sun, taking 6 hours to pass across from East to West (arrows indicate the entry and exit points). The weather was beautiful in the south of France near Carcassonne and I managed to observe the entire event.
Venus was just visible to the naked eye, with a suitable filter of course, whilst in transit - this picture, taken near the middle of the transit when Venus was furthest from the edge of the Sun, gives a good impression of what the view was like through an eclipse viewer. The position of Venus is rotated to the right compared to the picture above for this view is as it was seen by somebody standing on the ground, whereas the above picture, and all the others, are taken with an equatorial telescope drive so celestial north points straight up.
I had hoped that the Hydrogen Alpha filter would have allowed Venus to have seen silhouetted against the Sun's red atmosphere (the chromosphere) before entering the main part of the Sun (the chromosphere). However, the Sun's activity appeared low so there was little for it to be seen against, so I did not properly succeed in this.
Instead, the first time all of Venus could be seen was at 2nd contact, when the silhouette of Venus just breaks away from the edge of the Sun and is seen completely against the bright photosphere. There was much talk of the so-called "Black Drop" effect, when a piece of Venus appears to remain attached to the blackness beyond the edge of the Sun. However, there is no sign of it here, and neither did I see it at 3rd contact - maybe it is a phenomenon of visual observing only, which disappears when multiple webcam images are stacked and sharpened.
Click here for a full account with more pictures and timelapse animations.
You may also be interested to compare the size of Venus with the very much smaller, and further away, Mercury. This I caught in a similar transit, the first for 30 years, in May 2003.
Picture added on 10 June 2004