exports_controller.rb

  def download
    @email = User.emails.concat(Organization.emails).compact!
    outfile = "members_" + Time.now.strftime("%D") + ".csv"
    unless @email.blank?
      data = CSV.generate do |csv|
        csv << ['Email']
        @email.each do |e|
          csv << [e]
        end
      end
    end
    send_data data, :type => 'text/csv; charset=iso-8859-1; header=present', :disposition => "attachment; filename=#{outfile}"
    flash.now[:notice] = "Export Complete!"
  end

Send a CSV without writing a file, gogogadget send_data!