Avatars represent you and your personality on the internet. Not only in Instant Messaging applications but also several blog applications and forums let you add comments and use a global avatar.
You can use your avatar to tell the cyberworld something about yourself. Use it to show how you want to be seen or how you feel at hat moment. But please stop using those awfull default avatars. I've seen to much rubber duckies, frogs and racecars.
So here are some idears for creating your personalized avatar.