jQuery is arguably one of the coolest pieces of JavaScript code on the planet. While i ignored it for a long time because i thought (based on the name) that it was an SQL library for Java, once i did look at it it blew my socks off. As JS developer Dave Methvin says:
"You start with 10 lines of jQuery that would have been 20 lines of tedious DOM JavaScript. By the time you are done it's down to two or three lines and it couldn't get any shorter unless it read your mind."
- Dave Methvin
That sounds like a lot of hype, but in my experience it is indeed true. i believe it so much, that i wrote a short article about it, called Open letter to non-Believers.

If you do any significant amount of browser-based JavaScripting (as opposed to embedded code, as in SpiderMonkey/SpiderApe), take a few minutes to look over jQuery. Then take a few hours to get comfortable with it. Then weep for all of the hours you've previously wasted writing tedious code to traverse and manipulate DOM trees.

Once you start working with jQuery you will probably find it easy to start writing your own plugins. My plugins are: