You already know that with a little URL-hacking, you can skip to a certain time in YouTube clip embedded on your blog. Now you can do the same within the comments on a clip at the YouTube site. The TechCrunch blog describes how:
To specify a point, append a tag to the end of your video link with the following syntax: "#t=1m45s" (you can change the numbers before the 'm' and 's' to edit the minutes and seconds, respectively).
If you don't have a blog and you don't comment on YouTube, but you do want to share just the good part of a clip, you want the TubeChop webapp. There you can enter a YouTube clip URL, set the start time and end time of the best part using a slider, and send the chopped clip URL via email or on your site. Thanks, Bill! Update: In addition to TubeChop, Splicd can trim a clip down to the best part, too.