If you are using Gravatar to avatar-ize your WordPress blog’s comment section, you might want to consider using Twittar. Twittar is a WordPress plugin that pulls avatar off from a commenter’s Twitter account by matching the email address.

Developed by Patrick of BlogStorm, Blogicon acts as an avatar system to be displayed in the comment section. Unlike MyBlogLog or Gravatar, the avatar will be self hosted by the comment author, and it will be displayed on all participating blogs.