Here's a cool tip for YouTube videos: you can press 1 to jump to 10% into the video, 2 if you want to jump to 20%, 3 for 30% and so on. If you press 0, you'll go back to the beginning of the video. This trick works on YouTube's site, as well as on the embedded videos.
YouTube offers many other useful shortcuts:
- left/right arrow - seek backward/forward 5 seconds
- ctrl+left/ctrl+right - seek backward/forward 10%
- home/end - seek to the beginning/last seconds of the video
- spacebar - play/pause when the seek bar is selected
Here's a video you can use to try the shortcuts (make sure that the seek bar has focus by clicking on the small play button):