Measure test coverage for Rails application

add to Rakefile:

require 'rcov/rcovtask'
namespace :test do  
  namespace :coverage do
    desc "Delete aggregate coverage data."
    task(:clean) { rm_f "coverage.data" }
  end 
  desc 'Aggregate code coverage for unit, functional and integration tests'
  task :coverage => "test:coverage:clean"
  %w[unit functional integration].each do |target|
    namespace :coverage do
      Rcov::RcovTask.new(target) do |t| 
        t.libs << "test"
        t.test_files = FileList["test/#{target}/*_test.rb"]
        t.output_dir = "test/coverage"
        t.verbose = true
        t.rcov_opts << '--rails --aggregate coverage.data -x "/.gem/"'
      end 
    end 
    task :coverage => "test:coverage:#{target}"
  end 
end

run tests: rake test:coverage
If you are using Hudson you can install "Ruby metrics plugin" Rcov plugin parses rcov html report files and shows it on hudson with a trend graph.