Gravatars allow users to configure an avatar to go with a users email address at a central location: http://gravatar.com. Gravatar-aware websites can then look up and display each user's preferred avatar, without having to handle avatar management. This makes life easy on the developer and the user only has to manage one central location.
First step is to get the plugin
ruby script/plugin install git://github.com/woods/gravatar-plugin.git
Now most user models have an email field so it's usually something like below to access their email.
If thats the case then to have the gravatar appear in your views it's simply
<%= gravatar_for @user %>
Thanks Scott A. Woods for a great and simple plugin.