On many blogs, when you respond to the author’s post, a little icon appears next to your comment. If you’ve never wondered about that icon, then it is probably a complicated geometric shape, with some sense of symmetry. For example:

Figure 1: an identicon. A representation of everything that makes you who you are, compressed into one square centimetre.
Your identicon is automatically generated from your computer’s Internet address. You don’t have much say over what it looks like.
However, if you sign up for a free Globally Recognized Avatar, or Gravatar, you can choose any photograph, likeness, or sketch as your personal online representative.

Figure 2: Logos, photos, doodles, and smiling mugs of a few Urban Yukon members.
Gravatars are used in preference to geometric identicons, so as soon as you set one up, it’ll be active. Gravatars are also tied to your e-mail address rather than you computer’s address, so your gravatar will follow you to any computer you choose to use.
So What?
Still not persuaded to get a gravatar? Here’s another reason:
When you post a comment on someone else’s blog, if it has a gravatar icon next to it, that’s a good sign that you are a real person and not an automated blog spamming robot.
Spam messages almost always have the default identicons. I go through my WordPress spam folder every now and then to see if any legitimate comments were falsely picked as spam. There are a lot of spam messages though, so I just go by the little icon. If it’s an identicon, I ignore it, but if it’s a gravatar, then I look more closely. I’ve rescued a couple of bona-fide comments that way.
So do your fellow bloggers a favour and get a gravatar.