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:
Use a photo
- show your own face
as is 
or photoshopped like this 
- show your kids or your family
- show your favorite vacation picture
Create your personal avatar
Choose your face, clothes and accessoires an create your own avatar at one of the sites described below:
- Planearium's SouthPark Studio (works great, my favorite)
my avatar on a crusade 
and the editor 
- StorTroopers.com, choose your wardrobe and get dressed ...
two samples
and the editor 
- Meez, Java runtime required and you have to sign up for an account. Lots of export formats but for some items you have to pay.
my meez headshot
and the editor 
After you created your avatar image you have to resize it to a size of 80x80 or 100x100 pixels.
When you have created your avatar you can store your avatar online at GRAvatar.
GRAvatar stores your globally recognized avatar and lets webblogs and webforums display your avatar when you place comments. Great initiative from Tom Werner.
DasBlog the weblog software used for this blog supports gravatars from version 1.9.
I recently upgraded to dasBlog version 1.9 so the comments can display your gravatar if you have one.
UPDATE: At the moment it's not possible to sign up at Gravatar. Extreme load, Gravatar 2.0 and the wellknown apologies for the inconvenience. A quote from the Gravatar weblog: " ... don’t worry, Gravatar is NOT going to shut down".