Some capistrano tasks (http://wendbaar.nl/blog/2010/04/automagically-tagging-releases-in-github/)

ask_for_branch.rb

set(:branch) do
  if :production == stage
    branch = Capistrano::CLI.ui.ask("#{`git branch`}\n\nWhich branch do you want to deploy?: ")
    raise "Error: The master branch cannot be deployed to production." if 'master' == branch
  else
    `git branch | grep ^* | sed s/\\*\\ //`.chomp # use current active branche
  end
end

capistrano_git.rb

namespace :deploy do
  after  "deploy:restart", "deploy:git:push_deploy_tag"
  before "deploy:cleanup", "deploy:git:cleanup_deploy_tag"

  namespace :git do

    desc "Place release tag into Git and push it to server."
    task :push_deploy_tag do
      user = `git config --get user.name`
      email = `git config --get user.email`

      puts `git tag #{rails_env}_#{release_name} #{revision} -m "Deployed by #{user} <#{email}>"`
      puts `git push --tags`
    end

    desc "Place release tag into Git and push it to server."
    task :cleanup_deploy_tag do
      count = fetch(:keep_releases, 5).to_i
      if count >= releases.length
        logger.important "no old release tags to clean up"
      else
        logger.info "keeping #{count} of #{releases.length} release tags"

        tags = (releases - releases.last(count)).map { |release| "#{rails_env}_#{release}" }

        tags.each do |tag|
          `git tag -d #{tag}`
          `git push origin :refs/tags/#{tag}`
        end
      end
    end
  end
end

tags_deployed_releases.sh

for i in `ls -d *`
do rev=`cat $i/REVISION`
echo "git tag production_$i $rev"
done