When I need a Ruby library I head to GitHub and use their search to see if there is a library for my need.
But when the problem I’m trying to solve is common and there is multiple choices, I head to The Ruby Toolbox. They list Ruby libraries under categories and show you relevant information for your choosing process. Information like number of watchers and forks at GitHub, number of downloads from RubyGems and when the last commit was made to the repository.
Usually this information tells you which library is most used, supported and trusted in the community.