Ask HN: Must have gems for Ruby on Rails application?

[blohs]

I am trying to compile list of must have gems for development, testing and production environment for developing and maintaining Ruby on Rails app.

Format: [environment]:[gem]:[short description]

Thanks!

[blohs]

test : parallel_tests : ParallelTests splits tests into even groups (by number of lines or runtime) and runs each group in a single process with its own database.

[blohs]

development : bullet : Identify and kill n+1 queries, improves performance.