Rake taks for finding unused methods in your rails app. A bit hacky and slow, but seems to work for most cases!

gistfile1.rb

namespace :cleaner do
  task :find do
    models = Dir.glob("app/models/**/*")
    all_files = Dir.glob("app/**/*")
    models.each do |model|
      if File.file?(model)
        File.open(model).lines.each do |line|
          if line=~ /def ([a-z0-9._?!]*)/
            method_name = line.match(/def ([a-z0-9._?!]*)/)[1]
            method_name.sub!(/self./, '')
            use_count = 0
            all_files.each do |file|
              if File.file?(file)
                begin # Hack to handle binary files. Should probably do something else here
                  file_count = File.readlines(file).grep(Regexp.new(method_name)).size
                rescue
                end
                if file_count && file_count > 0
                  use_count = use_count + file_count
                end
              end
            end
            if use_count == 1
              puts "#{method_name} is a candidate for deletion"
            end
          end
        end
      end
    end
  end
end

Another revision. Haven't run this code, so I might've messed something up here: https://gist.github.com/strand/5062975