Most of my recommendations are performance oritented but I'll be happy to hear about Gems that can help in other domains.
PGHero https://github.com/ankane/pghero , great to keep track of your database , get index recommendations etc.
Scout Gem/Addon (Heroku , don't know how/if they work outside) of it , again great to keep track of N+1 issues , memory spikes and more.
memory_profiler https://github.com/SamSaffron/memory_profiler , great tool to analyze and compare your memory footprint.
Typehoeus https://github.com/typhoeus/typhoeus , multi-threading HTTP/HTTPS/SOCKS request. VERY easy to use for anything you need. Highly recommended.
Ox https://github.com/ohler55/ox , THE BEST XML parser for Ruby. Saved my life :)
Oj https://github.com/ohler55/oj , same author as Ox , probably the best JSON parser too.
ActiveRecord-Import https://github.com/zdennis/activerecord-import , bulk INSERT / UPDATE operations. Can save a HUGE amount of time.
bootstrap_sb_admin_base_v2 https://github.com/dreamingechoes/bootstrap_sb_admin_base_v2 , a Bootstrap based admin layout not the prettiest but works well and is very easy to setup.
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