YouTube started to roll out a few features that will make it easier to preview videos and to quickly jump to a certain scene. Just mouse over the seek bar and you'll see a thumbnail of the frame you've selected. The thumbnail is updated almost instantly when you pick a different frame.
This feature is obvious and you'll certainly notice it. There's also a way to preview multiple frames: just drag the handle along the seek bar and YouTube will "show a filmstrip of thumbnails of previous and upcoming scenes".
For movies and other videos longer than 90 minutes YouTube added a more advanced feature: a second seek bar that lets you preview one and a half minutes of video one second at a time.
These features aren't available to everyone yet, they'll be slowly rolled out in the coming weeks. For now, they only work in the Flash player. If you want to try them, they're enabled for videos longer than 90 minutes. Here's one of them: