Additional gems are listed here and here.
You are welcome to suggest your own!
- Active Record Nesting: Awesome nested set | Ancestry | Acts as tree | Closure Tre | Better nested set
- Active Record Soft Delete: rails3_acts_as_paranoid | paranoia | Acts as paranoid | Acts as archive | Permanent records
- Active Record Sortables: Acts as list | ranked-model | Sortable | Resort
- Active Record User Stamping: Userstamp | User stamp | Acts as audited | Record activities | Acts as loggable
- Pagination: will_paginate | Kaminari | sorted | ajax_pagination | Paged scopes
- Rails Comments: juvia | Acts as commentable with threading | Acts as commentable | Disqus | opinio
- Rails DB Bootstrapping: Seed-fu | Populator | seedbank | Bootstrapper | Db-populate
- Rails Ratings: Ajaxful-rating | ajaxful_rating | letsrate | Acts as rateable | mongoid_rateable
- Rails Search: Sunspot | sunspot_rails | Tire | Thinking-sphinx | Rsolr
- Rails Tagging: Acts-as-taggable-on | Is taggable | rocket_tag | Acts as taggable redux | Acts as taggable on steroids
- Background Jobs: Resque | Delayed job | sidekiq | Bunny | Beanstalkd
- Scheduling: Whenever | resque-scheduler | Rufus-scheduler | Clockwork | Craken
- Code Metrics: SimpleCov | Rails best practices | Rcov | rubocop | Cane
- Asynchronous E-Mail: Resque Mailer | Mail queue | Mailhopper | Resque Mail Queue
- CRM Apps: Fat free crm | Mongo crm | railscrm | myitcrm2 | Cohort
- E-Mail Preview: Letter Opener | MailCatcher | mail_view | Rails Email Preview | sanitize_email
- E-Mail Processing: SendGrid | Mailman | Fetcher | griddler | newman | Mail fetcher
- E-Mail Sending: Action Mailer | Mail | Pony | Tmail | capistrano_mailer
- Forum Systems: forem | Altered beast | thredded | forum_monster
- Inline CSS for E-Mail: premailer | Roadie | mail_style | inline-style | premailer-rails3
- Blog Engines: Octopress | Publify | Toto | blogit | Serious
- Content Management: Refinery CMS | BrowserCMS | Locomotive | Radiant | Comfortable Mexican Sofa
- Static Website Generation: Jekyll | Middleman | high_voltage | Nanoc | stasis
- Wiki Apps: Gollum | Git-wiki | Instiki | irwi | Olelo
- CSS with Ruby: Sass | Compass | sass-rails | bootstrap-sass | twitter-bootstrap-rails
- Font-Face Servers: Fontue | Fistface
- CSV Parsers: FasterCSV | smarter_csv | postgres-copy | ccsv | csv_pirate
- Redis Clients: Redis Ruby | Redis Store | Redis Objects | Ohm | redis_orm
- Benchmarking: method_profiler | rbench | bench_press | better-benchmark | Tach
- CLI Option Parsers: highline | Slop | cocaine | mixlib-cli | Trollop
- Configuration Management: dotenv | settingslogic | Figaro | dotenv-rails | configuration
- git Tools: Grit | githug | Ruby/Git | rugged | gitolite
- irb Tools: Dotfiles | Awesome print | Hirb | wirble | pry-rails
- Documentation Generators: RDoc | Yard | Sdoc | DocumentUp | Rocco
- Gem Doc Viewers: Bdoc | Sinatra-rubygems | Docbox
- PDF Generation: Prawn | Pdfkit | Wicked pdf | Wkhtmltopdf | Prawnto
- PDF Processing: pdf-reader | pdf_info
- Reports & Spreadsheets: spreadsheet | axlsx | roo | Ruport | google-spreadsheet-ruby
- Online Shops: Spree | RoR eCommerce | Invoicing | Shippinglogic | nimbleshop
- Payments: Active Merchant | braintree | Stripe | paypal_adaptive | paypal-express
- Subscription Management: Freemium | jackpot | fastspring-saasy | SubscriptionFu
- Graphing: Lazy high charts | Googlecharts | Gruff | Open flash chart | Tufte-graph
- Image Processing: chunky_png | RMagick | MiniMagick | Image science | image_sorcery
- QR Codes: rqrcode | rqrcode-rails3 | rqrcode_png | zxing | zbar
- Visualizing Data: Raphael | ruby-graphviz
- Video Streamio FFMPEG
- Breadcrumb Builders: breadcrumbs_on_rails | crummy | gretel | breadcrumbs | ariane
- Form Builders: Simple Form | Formtastic | nested_form | cocoon | twitter_bootstrap_form_for
- Markup processors: Markup | Redcarpet | RedCloth | vkhater-redcarpet | Rdiscount
- Microformats: Mofo | Microformats helper | Prism
- Rails Menu Builders: Simple-navigation | tabs_on_rails | Semantic-menu | Navigasmic | active_link_to
- Rails Wizards: Wicked | Actsaswizard | Wizardly | Wizard | Partially valid
- Syntax Highlighting: coderay | rouge | Albino | Jquery-syntax | Harsh
- Table Builders: datagrid | wice_grid | tabletastic | table_cloth | table_for_collection
- Template Engines: erubis | Tilt | Haml | Slim | Liquid
- JavaScript Tools: Uglifier | coffee-script | gon | opal
- App Instrumentation: New Relic RPM | dashing | better_errors | fnordmetric | bullet
- Exception Notification: Errbit | Airbrake | Exception notification | Exceptional | Honeybadger
- Log Analysis: logstash | Graylog2 | request-log-analyzer | central_logger | ntail
- Logging: log4r | Logging | gelf | yell | mongodb_logger
- Server Monitoring: God | Bluepill | amon | sensu | Outpost
- Bundler Tools: rubygems-bundler | Appraisal | guard-bundler | spitball | noexec
- Dependency Management: Bundler | berkshelf | librarian | Braid | Isolate
- Gem Hosting: geminabox | stickler
- Packaging Systems: Rubygems | Rip | Rpg | Coral
- Packaging to Executables: Ocra | releasy | Crate
- Continuous Integration: Travis | Cruisecontrol.rb | Cijoe | Jenkins.rb | Integrity
- Deployment Automation: Capistrano | Heroku | mina | Moonshine | git-deploy
- Web servers: Thin | Unicorn | Passenger | Puma | Pow
- Rails Activity Feeds: public_activity | TimelineFu | Pfeed | streama | unread
- Rails Admin Interfaces: ActiveAdmin | Rails Admin | ActiveScaffold | Typus | Streamlined
- Rails App Templates: rails3-application-templates | rails_apps_composer | Rails-templates | Suspenders | Bigoldrailstemplate
- Rails File Uploads: Paperclip | Carrierwave | Attachment fu | Dragonfly | Imagery
- Rails Permalinks & Slugs: FriendlyId | Stringex | Babosa | Permalink fu | ObfuscateId
- Rails Presenters: Draper | Cells | Apotomo | Active presenter | ActiveDecorator
- Rails Subdomains: Subdomain-fu | Subdomain routes | Subdomain account | Sso what
- Recommendation Engines: recommendify | recommendable | coletivo | Slope One
- Reputation Engines: activerecord-reputation-system | merit | honor | recognition
- User Messaging Systems: mailboxer | rails-messaging | acts-as-messageable | maktoub | inboxes
- Encryption: Attr encrypted | Strongbox | crypt_keeper | symmetric-encryption | Sentry
- Rails Authentication: Devise | OmniAuth | Authlogic | Restful-authentication | Sorcery
- Rails Authorization: Cancancan | Declarative authorization | rolify | pundit | Acl9
- Rails Captcha: Recaptcha | Negative-captcha | Simple-captcha-mirror | galetahub-simple_captcha | humanizer
- Security Tools: brakeman | Loofah | Tarantula | Xss terminate | Rails xss
- Spam Detection: Rakismet | Defender | Defensio-ruby | Akismetor | Rdefensio
- A/B Testing: Vanity | Split | seven_minute_abs | bandit | ABingo
- Calendars: Event calendar | ri_cal | Calendar helper | icalendar | Weekly builder
- Geocoding & Maps: Geocoder | Geokit | gmaps4rails | RGeo | Geokit-rails
- I18n: I18n | i18n-js | R18n | http_accept_language | Translate routes
- Natural Language Date Parsing: Chronic | Chronic Duration | Nickel | Tickle | kronic
- Recurring Events: Ice cube | Recurrence | Schedule fu | Runt | Recurring event
- API Builders: Grape | rabl | jbuilder | active_model_serializers | acts_as_api
- API Clients: Twitter | koala | fb_graph | Linkedin | youtube_it
- Atom & RSS Feed Parsing: Feedzirra | Simple-rss | Feed-normalizer | ratom | rfeedparser
- HTML parsing: Nokogiri | Hpricot | Libxml-ruby | Scrubyt | Scrapi
- HTTP clients: Rest-client | Httparty | faraday | multipart-post | Excon
- JSON Parsers: json | multi_json | yajl-ruby | json pure | crack
- URL Rewriting: rack-rewrite | Refraction | redirect
- User Agent Detection: useragent | browser | mobylette | agent_orange | user-agent
- Web Analytics: gattica | gabba | rack-google-analytics | Garb | google-analytics-rails
- Web App Frameworks: Sinatra | Padrino | Cramp
- Web Content Scrapers: Pismo | data_miner | metainspector | cobweb | sinew