Google just launched a new version of Google Earth (news, download) from which you gaze up from the surface of the planet, not just down on it. It's a good way to see which stars and planets are over your home, right now. You can also check out a rich database of Hubble Space Telescope images that is overlaid on the celestial map.
The new Google Earth has a lot of additional education and reference material linked to it, pulled in from the Net as needed. The program is a great way to learn about the night sky. It has two big limitations, though: your point of view is limited to Earth (you can't see the stars from other locations) and you have an extremely limited control of time. If you want to see where the planets were on your birthday, for example, you can't.
If your curiosity about the universe bumps into Google Earth's edges, I'd recommend also checking out these two applications:
Celestia (download) is a 3D simulation of the galaxy. Its special power is not its imagery (Google's is better, although Celestia does a good job with planets and asteroids in our solar system), but rather that you can zoom in on any object in the program's database and see the galaxy from that perspective. You can also see the position of stars at any point in time and can control the rate of time's passage to see how objects move over the millennia.
Stellarium (download) is a gorgeous planetarium for your computer. Its sky and star visuals are a lot more compelling then either Google's or Celestia's, although Stellarium does not have detailed Hubble overlays. Like Google, it's Earth-bound (you can't move your point of reference), but like Celestia, it gives you good control over time so you can see the heavens wheel about. My favorite feature is that it will also overlay constellation lines from other cultures (Chinese, Inuit, and so on); Google only shows the Western constellations.
There are also Web-based online planetaria. They have good data, but they don't give you the smooth visual controls that the downloadable applications do. See Sky-map.org, WikiSky (review), and YourSky. You can control a powerful stargazing telescope yourself via the Web at the pay site Slooh (review). There are also astronomy gadgets covered over on our gadget blog, Crave.
Finally, if the real galaxy doesn't appeal to you, check out the collaborative work of fiction called Galaxiki. Be advised that it was named one of the "Five stupidest start-ups of the summer" by Valleywag.