I've always been a fan of the Magic Eightball. I've found it to give very good advice at least 50% of the time. This is an astonishingly higher accuracy rate than most people who give me advice. In that spirit, Magic Eightball Everywhere:
Here's an online magic eightball. It's a Web app. You can also see what other people are asking, although I found that this feature was not for the faint of heart and could be downright depressing. People ask all sorts of sappy questions like; "Does she ever think of me now?" It just breaks my little heart.
More impressive is The Public Eightball. It's an actual Magic Eightball that is hooked to a Lego Mindstorm-based apparatus that shakes the ball for you and displays the result in a Webcam image. Unfortunately, it seems to currently be off kilter, and you can't see the result.
If you don't find either of these to your liking, Yahoo actually has a portal category devoted to just online Magic Eightball Apps. A more thorough listing, with reviews, is here. The same site has a listing of the actual Eightball answers. You can find a scientific dissection of an Eightball here.
If you are running Mac OS X, this dockling will allow you to have a Magic Eightball in your dock.
If you are running Windows, this app will let you have a Magic Eightball in your system tray.
I'm ashamed to admit it, but I could not find a standalone Java app or a Linux app that simulates the Magic Eightball. If you know of one, please email me so that I can add it to this post. I'm certain that they are out there somewhere.
This site seems to have the best little history of the Eightball that I could find, but it's not very in-depth.