Here's an oldy but goody: Video of astronauts from Apollo 15 proving Galileo's hypothesis that, in the absence of wind resistance, two objects will fall at the same rate regardless of how much they each weigh.
There are a number of NASA pages centered around this video:
Click here for Quicktime movies and description of the experiment from the Apollo 15 Preliminary Science Report.
Click here for transcript of the audio feed from before, during, and after what you see in this video.
Click here for an in-depth explanation of Galileo's Equivalence Principle and an experiment you can try at home.
Click here to find out why scientists are still putting the Equivalence Principle to the test, measuring it with increasingly sensitive equipment to make sure it holds true.
