These are some of my (Ryan Bates) favorite gems to use for various tasks:
- JavaScript: jQuery with jquery-rails
- Pagination: Kaminari
- Testing: RSpec, Cucumber and Capybara
- Factories: Factory Girl
- Authentication: Nifty Generators and Omniauth
- Authorization: CanCan
- File Attachments: Carrierwave
- HTML/XML Parsing: Nokogiri
- Scheduling: Whenever
- Monitoring: God
- Server: Passenger
For everything not listed here, I depend on:
https://www.ruby-toolbox.com/categories
It does a decent job in showing which gems are actively supported and popular.