Below are a list of Gems have found to be useful as I was learning rails:
FactoryGirl automates the process of model creation for the purpose of testing.
Launch allows you to open a webpage in testing, good for debugging tests
Annotate creates annotations for models in the model file based on the database. Has to be run after each new migration.
Faker fills in mock data to populate the database for web testing
Spring is a rails environment pre-loader for quicker rake tasks and tests.