_filter_organisations.html.erb

<% @organisations_by_letter.each do |letter| -%>
  <li><%= link_to_unless(letter == params[:id], letter, organisation_url(letter)) do |name|
    link_to(name, organisation_url(letter), :class => "active")
    end %></li>
<% end -%>

organisation.rb

class Organisation < ActiveRecord::Base
  # provided through utility_scopes gem http://github.com/yfactorial/utility_scopes/tree/master
  # Rails 2.2 EDGE has default scopes
  ordered_by "name"
  # default_limit '1000'
  
  named_scope :letter, lambda { |letter| {:conditions => conditions(letter) }

  def self.conditions(letter)
    if letter == "Other"
      "name REGEXP '^[[:digit:]]|^[[:space:]]|^[[:punct:]]'"
    elsif !letter.blank?
      ["name like ?", letter + "%"]
    end
  end
end

organisation_controller.rb

class OrganisationController< ApplicationController
  def index
    @organisations_by_letter = ("A".."Z").to_a << "Other" # to generate the letters list
    params[:id] = "A" if params[:id].nil? || !@organisations_by_letter.include?(params[:id])
    @organisations = Organisation.ordered.letter(params[:id])
  end
end