simple gravatar from email

_gravatar.html.haml

%a{:href => user_path(@user)}
  = image_tag(@user.gravatar_url({:size => 250,
    :default => default_gravatar,
    :width => "200px",
    :height => "200px"}))

gravatar_helper.rb

module GravatarHelper

  def default_gravatar
    'mm'
  end

  # image_url

  # Rails currently only provides path helpers for assets. In this application
  # the path also includes the asset host config, which is currently set up
  # for multiple asset hosts. However, the asset host setting doesn't include
  # the scheme / protocol since they are designed to be protocol agnostic.
  # As a result, the URLs don't work when referenced externally, such as when
  # passed as the 'default' parameter value in the gravatar URL. This helper
  # constructs a full URL with the scheme. 
  def image_url(image, opts = {})
    %Q|#{request.protocol.gsub(%r{//$}, '')}#{image_path(image)}|
  end

end

user.rb

class User < ActiveRecord::Base

  # ...

  def gravatar_url(options = {})
    options = {:size => 80}.merge(options)
    hash = Digest::MD5.hexdigest(self.email.downcase)
    gravatar = "http://www.gravatar.com/avatar/#{hash}?size=#{options[:size]}"
    if options[:default].present?
      "#{gravatar}&default=#{URI.escape(options[:default])}"
    else
      gravatar
    end
  end

end