The International Space Station and the Shuttle, that is. Have you seen it? It's highly visible from now until Wednesday.
Here's a link to the NASA site that will let you choose your state and city so you can see when and where to look: ISS/Shuttle tracking.
It really moves across the sky. It's a bright white ball of light. You'll know it when you see it because it moves faster than anything you've seen crossing the sky. Go out and take a peek! You'll be glad you did.