I tried to avoid some, well, really obvious choices (like rspec, mocha, sidekiq, devise, resque, carrierwave, etc. - you get the idea) so maybe you fill find that one precious, very special thing that you will love ;-). Here we go.

• active_link_to - one simple method for your views that will handle all the logic.

• active_decorator - object-oriented view helper for Rails 3 and Rails 4. VERY useful when working with complex views.

• annotate - annotate your models and specs, display columns&indexes right within your ruby class.

• ambry - database and ORM replacement for static models / small datasets. I like to use it from time to time.

• better_errors - essential dev gem for your rails environment.

• brakeman - scan you Rails app for potential vulnerabilities.

• bundler-audit - patch-level verification for Bundler, make sure your app’s dependencies are secure.

• charlock_holmes - ask Holmes which encoding is used.

• counter_culture - make some complex counters with ease.

• devise-async - universal plug&play async backend for devise mailer.

• email-spec - just a great addition to rspec for testing emails.

• excon - my favorite HTTP library for Ruby. Fast & reliable, simply love it.

• ffaker - fake your test data, but faster.

• identity_cache - cache your Rails models with ease (but look out for gotchas, caching ain’t easy)

• lol_dba - can I haz db indexes? Scan & see if you’re missing some indexes maybe?

• merit - swiss army knife when it comes to points, badges & rankings. Gamify your whole app with just one gem.

• paranoia - soft delete solution for Rails. Tiny & reliable.

• redcarpet - one of the best Markdown parser I know.

• rolify - roles solution for Rails. Plug & Play.

• spring - shipped with Rails 4.1, basically no more need for guard, zeus or spork.

• split - A/B testing? No problem.

• slim - you probably know it. You may love or you may hate it. I chose <3.

• thinking-sphinx - integrate Sphinx right way.

• timecop - because testing time can be tricky, make it less tricky.

• websocket-rails - use the power of HTML5 websockets! If you need it, of course.