Hi All,

The best gem or plugin for rails application as below,

1. Geo location or google map - https://github.com/alexreisner/geocoder

2. Rails Authentication - https://github.com/plataformatec/devise

3. Twitter API - https://github.com/sferik/twitter

4. File Uploads - https://github.com/thoughtbot/paperclip

5. Deployment Automation - https://github.com/capistrano/capistrano

6. E-Commerce - https://github.com/spree/spree

7. Rails Admin Interfaces - https://github.com/gregbell/active_admin

8. Rails Pagination - https://github.com/mislav/will_paginate

9. Rails Search - https://github.com/sunspot/sunspot

10. Rails Ratings/votings - https://github.com/edgarjs/ajaxful-rating

11. Web servers - https://github.com/macournoyer/thin

12. Exception Notification - https://github.com/rails/exception_notification

13. JavaScript Frameworks - https://github.com/jquery/jquery

14. Rails Captcha - https://github.com/ambethia/recaptcha

15. Rails Comments - https://github.com/jackdempsey/acts_as_commentable

16. Scheduling tasks - https://github.com/javan/whenever

17. facebook API - https://github.com/mmangino/facebooker

18. Sitemap generator - https://github.com/kjvarga/sitemap_generator

19. JavaScript and CSS Asset Compression for Production Rails Apps - https://github.com/sbecker/asset_packager

20. Rails Permalinks & Slugs - https://github.com/norman/friendly_id

21. E-Mail Sending - https://github.com/mikel/mail

22. Markup processors (html convert) - https://github.com/rtomayko/rdiscount/

23. Rails Captcha - https://github.com/ambethia/recaptcha

24. PDF Generation - https://github.com/sandal/prawn

25. Ruby Version Management - https://github.com/wayneeseguin/rvm

26. Background Jobs - https://github.com/collectiveidea/delayed_job

Useful Links

1. Ruby regular expression editor - http://rubular.com/

2. Convert Html Snippets to Haml Snippets - http://html2haml.heroku.com/

3. Convert CSS Snippets to Syntactically Awesome StyleSheets code - http://css2sass.heroku.com/

4. displaying images, html content and multi-media in a Mac-style "lightbox" that floats overtop of web page - http://fancybox.net/

5. The online event registration and ticketing tool - https://www.amiando.com/

6. Epoch & Unix Timestamp Conversion - http://www.epochconverter.com/

7. Email Delivery - http://sendgrid.com/

8. Online image converter - http://www.go2convert.com/ 

9. Web Page Validation - http://www.w3schools.com/site/site_validate.asp

10. Web technology information profiler tool - http://builtwith.com/