Lets say there is a YouTube video you want to start after 1 min and 20 secs.

Then if the URL of the video is for example:

Code:
http://www.youtube.com/watch?v=ABCDXyZyHSE
The you should make the URL

Code:
http://www.youtube.com/watch?v=ABCDXyZyHSE#t=1m20s
The YouTube video will start directly from a point after 1 min 20 secs.

Hope you like the trick!!!