15 years ago, the world watched as broken-apart comet Shoemaker-Levy 9 impacted Jupiter's atmosphere in a series of event in July 1994. Well, a similar event seems to have occurred in the last day in Jupiter's south polar region. A new dark spot, similar in size and color to the Shoemaker-Levy 9 impacts was seen by ground-based observers on July 19.
Hopefully more observations will be acquired over the next few days to help confirm this discovery, but it looks quite plausible to me :)
The image at left was captured by Anthony Wesley on 19th July 2009 at 1554UTC from Murrumbateman Australia. The south pole is up, north pole down. The impact site is near the central meridian about an eighth of the way down.
EDIT 07/19/2009 9:23 PM: Wesley's website has been slashdotted so he has mirrored the page to another server. So if you are having trouble accessing the link above, check out http://jupiter.samba.org/