1. Active Merchant
用于电子商业，处理电子支付事务。
Active Merchant is an extraction from the e-commerce system Shopify . Shopify's requirements for a simple and unified API to access dozens of different payment gateways with very different internal APIs was the chief principle in designing the library.
链接：
http://www.activemerchant.org/
https://github.com/akitaonrails/active_merchant
2. acts_as_list
This acts_as extension provides the capabilities for sorting and reordering a number of objects in a list. The class that has this specified needs to have a +position+ column defined as an integer on the mapped database table.
链接：
https://github.com/swanandp/acts_as_list
3. acts_as_state_machine
状态机
详细实例介绍：http://www.ibm.com/developerworks/cn/java/j-cb03137/
4. acts_as_tree
Specify this +acts_as+ extension if you want to model a tree structure by providing a parent association and a children association. This requires that you have a foreign key column, which by default is called +parent_id+.链接：https://github.com/amerine/acts_as_tree
5. annotate_models
Annotate ActiveRecord models as a gem
https://github.com/ctran/annotate_models
6. AssetPackager
JavaScript and CSS Asset Compression for Production Rails Appshttps://github.com/sbecker/asset_packager
7. attachment-fu
attachment_fu is a plugin by Rick Olson (aka technoweenie <http://techno-weenie.net >) and is the successor to acts_as_attachment. attachment_fu facilitates file uploads in Ruby on Rails. There are a few storage options for the actual file data, but the plugin always at a minimum stores metadata for each file in the database. There are four storage options for files uploaded through attachment_fu: File system Database file Amazon S3 Rackspace (Mosso) Cloud Files 链接：https://github.com/technoweenie/attachment_fu 8. authlogic Authlogic is a clean, simple, and unobtrusive ruby authentication solution. 链接： https://github.com/binarylogic/authlogic 9. auto_complete 搜索框里面实现很酷的自动补齐。 https://github.com/david-kerins/auto_complete
10. calendar_date_select
11. clearance
Rails authentication with email & password.
链接：https://github.com/thoughtbot/clearance
email + confirmation + password + forget = authentication
12. delayed_job
Database based asynchronously priority queue system -- Extracted from Shopify
Delated_job (or DJ) encapsulates the common pattern of asynchronously executing longer tasks in the background.
It is a direct extraction from Shopify where the job table is responsible for a multitude of core tasks. Amongst those tasks are:
sending massive newslettersimage resizinghttp downloadsupdating smart collectionsupdating solr, our search server, after product changesbatch importsspam checks链接：https://github.com/collectiveidea/delayed_job
13. devise
Flexible authentication solution for Rails with Warden.
Devise is a flexible authentication solution for Rails based on Warden. It:
Is Rack based; Is a complete MVC solution based on Rails engines; Allows you to have multiple roles (or models/scopes) signed in at the same time; Is based on a modularity concept: use just what you really need.It’s composed of 12 modules:
链接：https://github.com/plataformatec/devise
13. exceptional
https://github.com/adamwiggins/exceptional
14. exception_notification
The Exception Notifier plugin provides a mailer object and a default set of templates for sending email notifications when errors occur in a Rails application.
15. geokit-rails
基于地理位置的rails应用：Provides location-based goodness for your Rails app. Requires the Geokit gem.
Geokit provides key functionality for location-oriented Rails applications
链接：https://github.com/andre/geokit-rails
16. HoptoadNotifier
This is the notifier gem for integrating apps with Hoptoad.
When an uncaught exception occurs, HoptoadNotifier will POST the relevant data to the Hoptoad server specified in your environment.
链接： https://github.com/thoughtbot/hoptoad_notifier
17. in_place_editing
InPlaceEditing
链接：https://github.com/amerine/in_place_editing
18. paperclip
Paperclip is intended as an easy file attachment library for ActiveRecord. The intent behind it was to keep setup as easy as possible and to treat files as much like other attributes as possible. This means they aren't saved to their final locations on disk, nor are they deleted if set to nil, until ActiveRecord::Base#save is called.
Easy file attachment management for ActiveRecord
链接: https://github.com/thoughtbot/paperclip
19. ssl_requirement
SSL requirement adds a declarative way of specifying that certain actions should only be allowed to run under SSL, and if they're accessed without it, they should be redirected.
链接: https://github.com/retr0h/ssl_requirement
20. thinking-sphinx
A Ruby connector between Sphinx and ActiveRecord.
链接：http://freelancing-god.github.com/ts/en/
Railscasts： http://railscasts.com/episodes/120-thinking-sphinx
21. will_paginate
Adaptive pagination plugin for web frameworks and other applications
链接：https://github.com/mislav/will_paginate/wiki
Part II Replacement parts
1. capistrano
Capistrano is a utility and framework for executing commands in parallel on multiple remote machines, via SSH. It uses a simple DSL (borrowed in part from Rake, rake.rubyforge.org/) that allows you to define tasks, which may be applied to machines in certain roles. It also supports tunneling connections via some gateway machine to allow operations to be performed behind VPN’s and firewalls.
2. cruisecontrol.rb
持续集成工具
CruiseControl.rb is a continuous integration tool written in, and for, Ruby. It makes it easy to set up automated builds of your software and to find out about tests that break on the fly.
3.datamapper
Ruby的对象关系映射器（ORM）DataMapper
4. faraday
????
5. rspec and cucumber
Behaviour Driven Development framework
6. sinatra
Sinatra is a DSL for quickly creating web-applications in Ruby with minimal effort.
7. Webrat
If you prefer writing integration tests in ruby instead of Cucumber's plain english, consider interacting with Webrat directly as I show in this episode.
8. sequel
Sequel 是轻量级的 Ruby 数据库访问工具
9. PG
?
10. memcache-client
Rails 自带memcache-client ，作为操作memcached缓存的客户端
别人的总结：
http://hlee.javaeye.com/category/58661?show_full=true
