Deep Impact
On July 4th at 06:00 UT, part of the spacecraft Deep Impact will collide with comet 9P/Tempel 1. The date isn't really a coincidence; NASA plan to have a large fireworks show for the US 4th July celebrations. Actually, before you start thinking that we will be hit by debris, don't worry; the comet is a long way from Earth. As it only reaches magnitude 9.7 before impact, I guess that you would still need a telescope to see it even if it brightens by a few magnitudes.
Unfortunately, those of us in the UK will be on the wrong side of the planet to get a good view with our telescopes. The comet will appear close to the star Spica, in the constellation Virgo, so it won't rise until around 16:00 UT. As sunset isn't until around 20:40, it means that we will get our first glimpse at least 14 hours after impact. Luckily, we have the Internet so that we can see live video and other people's pictures.