1. View high quality videos
Youtube gives you the option to switch to high quality videos for some of the videos, however you can check if a video is available in high quality format by appending &fmt=18(stereo, 480 x 270 resolution) or &fmt=22(stereo, 1280 x 720 resolution) for even higher quality.

2. Embed Higher Quality Videos
While the above trick works for playback, if however you want to embed hig quality videos you need to append &ap=%2526fmt%3D18 and &ap=%2526fmt%3D22 to the embed url.

3. Cut the chase and link to the interesting part
Linking to a video where the real action starts at 3 minutes 22 seconds, wondered if you could make it start at 03:22? You are in luck. All you have to do is add #t=03m22s (#t=XXmYYs for XX mins and YY seconds) to the end of the URL.

4. Hide the search box
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The search box appears when you hover over an embedded video. To hide the search box add &showsearch=0 to the embed url

5. Embed only a part of Video
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Just append &start=30 to skip first 30s of the video. In general you can modify the value after start= to the number of seconds you want to skip the video for.

6. Autoplay an embedded video
Normally when you embed a Youtube video and load the page, the player is loaded and it sits there waiting for you to hit the play button. You can make the video play automatically by adding &autoplay=1 to the url part of the embed code.

7. Loop an embedded video
Append &loop=1 to make the video start again without user intervention after it reaches the end.

8. Disable Related Videos
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Publishing your content in the form of Youtube video? Dont want people to see other peoples content that may be related but may as well be in competition to you? Just add &rel=0 to the end of the url part of the embed code and you just turned off the related video suggestions!

9. Bypass Youtube Regional Filtering
Some videos are only available in certain parts of the world. Your IP Address is used to determine your location and then allow or deny access to the video. Change the url from [ Register or Signin to view external links. ] to [ Register or Signin to view external links. ]

10. Download Video
Although not inherently a youtube trick but useful all the same for downloading videos. Just change youtube to kickyoutube in the url of the video and it will take you to kickyoutube.com with all the options for downloading the video you were watching.