I saw Gravatar support implemented at Pete's blog and thought it was pretty cool - so I signed up and enabled it in my comment's plugin as well. I'm currently searching for a good default avatar to use, but for now their system's 1x1 px image is working just fine.

Gravatar is a globally recognizable Avatar system - your register an avatar associated with your email address at their site, and then anyone can look it up by taking an MD5 hash of your email address as your gravatar id. This is very simple in Ruby, just do:

require 'md5'

email = 'users_email@domain.com'
gravatar_id = MD5::new( email ).hexdigest
gravatar = '<img src="http://www.gravatar.com/avatar.php?gravatar_id=%s"/>' % gravatar_id

More details about URL parameters can be found at Gravatar's implementation page.