1. devise: Flexible authentication, install devise-async if using sidekiq
2. rspec: Behaviour Driven Development
3. capistrano: Executing commands in parallel on multiple remote machines, simplify and automate deployment
4. aasm: Adding finite state machines to Ruby classes. Try state_machine if you need support for multiple columns within the same model
5. tabs_on_rails: Creating tabs and navigation menus
6. faker: Generates fake data, very handy with factory_girl for tests
7. brice: Irb goodness for the masses
8. webmock: Stubbing and setting expectations on HTTP requests
9. sidekiq: Efficient message processing, Resque compatible
10. carrierwave: Flexible way to upload files
11. will_paginate: Pagination library, I use it with will_paginate-bootstrap as I use bootstrap most the time
12. whenever: Clear syntax for writing and deploying cron jobs

Which ones do you use yourself? Let me know over Twitter.

Update

Some more I got from you guys:

• better_errors: Better Errors replaces the standard Rails error page with a much better and more useful error page. It is also usable outside of Rails in any Rack app as Rack middleware.
• pry: An IRB replacement
• simple_form: Rails forms made easy.
• irbtools: Improves Ruby’s irb console. Unlike pry, you are still in your normal irb, but you have colors and lots of helpful methods. It’s designed to work out-of-the-box so there is no reason to not use it!
• nokogiri: Nokogiri (鋸) is an HTML, XML, SAX, and Reader parser. Among Nokogiri’s many features is the ability to search documents via XPath or CSS3 selectors.
• bundler: Bundler keeps ruby applications running the same code on every machine. Of course I’d use this in every single projects.
• kaminari: A Scope & Engine based, clean, powerful, customizable and sophisticated paginator for modern web app frameworks and ORMs. Seems like a nice replacement to will_paginate
• Active Admin: Framework for creating administration style interfaces.