This is an amazing new feature in iChat. You can show files, photos, slideshows, or movies while video-chatting with a friend or co-worker. During a video chat, simply drag any file, photo (or a collection of files or photos), or movie from your hard disk and drop it into the video chat window. Your buddy's image gets resized to the corner and the file or movie takes center place.
At the same time, iChat opens a file window on your screen that allows you to scroll through the same multi-page document as your buddy watches, or you can play or pause the movie, or show the next or previous image in the slideshow in the chat window. All this time, you are talking to and seeing each other!
You can even share an iPhoto album: After you start a video chat, go to the File menu and choose "Share iPhoto With iChat Theater...," then choose an iPhoto album from the window that appears. Amazing.