If you're in the western part of the United States, a Pacific island, or China make sure you check out the solar eclipse this weekend. During a solar eclipse, the Moon will pass between the Earth and the Sun, blocking a significant portion of the Sun as seen from the Earth. It's an annular eclipse because the Moon isn't large enough to block the entire Sun. Instead, the Sun will appear to be a crescent or ring of fire around the shadow of the Moon.
As always, do not stare directly at the Sun. It will damage your vision. Do not look at the sun through your sunglasses! They don't block enough light to protect your eyes. Likewise, pointing your digital camera at the Sun can damage its sensor.
The safest way to view an eclipse (or check out sunspots) is by making a basic projector. You'll get best results by projecting the sun onto a sheet of white paper using a pair of binoculars and a white sheet of paper, but it's also simple to make a pinhole projector. Space.com has a good guide for viewing eclipses and the Sun.
Because the Moon won't completely block the Sun on Sunday, it won't be unusually dark. Instead, expect a strange twilight effect during the eclipse, and be sure to look at the shadows cast during the eclipse. They should be unusually shaped, especially when the Sun is a crescent.
To find out when the eclipse starts and finishes in the US, you can grab a handy chart from NASA or check your local news. If you're outside the optimal viewing area, there will be online streams from several locations. I can't wait to see photos from ISS of the Moon's shadow crossing the globe.