The most popular service for avatars is Gravatar (short for globally recognized avatar), which has just relaunched as much easier and useful. All you have to do is sign up with your e-mail address, upload a photo (the site will crop and resize it for you), give it a rating (G, PG, R, X), and select it. Now, whenever you leave a comment on a site that supports Gravatars, your image will appear by your comment.
On my site I also use MyBlogLog, a blog community site, to assign avatars to comments. If you’re a member there, your avatar will appear by comments you make, as well as showing up in my recent readers sidebar.
You can have an avatar that will follow you around the blogosphere in just a couple minutes, so I hope you’ll give it a shot.
Blog hosts and those of us who have blogs we maintain ourselves often have to add avatar support. I use Word Press for my blogging softwars, and there’s a slick plugin, MyAvatars, that lets us use both Gravatars and MyBlogLog. I think it does add a lot to the blogging experience, so I hope to see it become increasingly common to see avatars working across different blog platforms.