My Guard-based build system for front-end assets – see my blog post at http://monospace.se/posts/guard-build-system

assets.yml

gzip_assets: off
javascript_compressor: uglifier

javascripts:
  javascripts/site.min:
    - javascripts/src/vendor/jquery.js
    - javascripts/src/lib/*.js
    - javascripts/src/site.js

stylesheets:
  stylesheets/site.min:
    - stylesheets/site.css

Gemfile

source :rubygems

# V8 JavaScript Engine (for Uglifier)
gem "therubyracer"

# SASS & Compass
gem "sass", "~> 3.2.0.alpha"
gem "compass"

# For concatenation/compression
gem "jammit"
gem "uglifier"

# Guard and plugins
gem "guard"
gem "guard-sass"
gem "guard-jammit"
gem "guard-livereload"

# Mac-specific stuff
if RUBY_PLATFORM =~ /darwin/i
  gem "rb-fsevent"
end

Guardfile

guard :sass,
  :input        => "stylesheets/src",
  :output       => "stylesheets",
  :line_numbers => true,
  :load_paths   => Dir.glob('**/**').find_all { |i| File.directory?(i) } << "#{Gem.loaded_specs['compass'].full_gem_path}/frameworks/compass/stylesheets"

guard :jammit,
  :config_path => "assets.yml",
  :output_folder => "." do
    watch(%r{(?:stylesheets|javascripts)(/.+)\.(?:css|js)}) { |m| m[0] unless m[1] =~ /\/\./ }
  end

guard :livereload,
  :apply_js_live => false do
    watch(%r{(.+)\.(?:html|php)}) { |m| m[0] unless m[1] =~ /^\./ || m[1] =~ /\/\./ }
    watch(%r{(?:stylesheets|javascripts)(/.+)\.(?:css|js)}) { |m| m[0] unless m[1] =~ /\/\./ }
  end