Here are some of the better Astronomy-based websites that we've found during our travels...
Astronomy Picture of the Day
A breathtaking shot of the Cosmos every single day, along with a dose of education to go with it.
Celestia
Three-dimensional "planetarium" software for your computer, allowing you to explore every known nook and cranny of space.  And best of all, it's FREE!
Huygens Landing on Titan
NASA's most recently-landed probe set down on the only other body in the solar system that has liquids on its surface: Saturn's largest moon, Titan.  Unlike Earth's water, however, the temperature on Titan is so cold that lakes and rivers of liquid methane and ethane dot its surface.  Watch as Huygens penetrates the moon's thick atmosphere, and lands in breathtaking detail.
esa Kids
Easy-to-read, visually stimulating astronomy sites geared towards kids!
NASA's Living in Space
Interactive Moon Phases
An excellent animated website shows what actually causes the Moon's phases, and how they change over the course of a month.
Solar System Collisions
KABOOM!  See what would really happen when Earth or any other planet in the solar system is struck by an impactor from outer space.  Alter the projectile's composition, diameter, velocity, and target.  Have fun, but realize that Earth is in danger of just such a collision!
Constellation Mythologies
The most detailed mythological interpretations surrounding every constellation in the night sky.  Great example of the universality of the study of astronomy!
More Constellation Mythologies